a substrate holder configured to horizontally hold a substrate from below;

a processing laser oscillator configured to oscillate a processing laser beam for processing the substrate;

a slider configured to move the substrate holder horizontally and move a processing irradiation point of the processing laser beam on an upper surface of the substrate held on the substrate holder;

a height measurement unit configured to measure a vertical direction position of the processing irradiation point;

a condensing lens configured to condense the processing laser beam from above to below the processing irradiation point;

an actuator configured to vertically move moves the condensing lens; and

a controller configured to control a vertical direction position of the condensing lens based on the vertical direction position of the processing irradiation point while moving the processing irradiation point along multiple dividing target lines on the upper surface of the substrate,

wherein the height measurement unit includes:

a coaxial laser displacement meter configured to measure a vertical direction position of a first measurement irradiation point, which is overlapped with the processing irradiation point in a vertical direction, by radiating, to the upper surface of the substrate, a first measurement laser beam having a different wavelength from the processing laser beam and a path same as the processing laser beam from a dichroic mirror provided in a path of the processing laser beam to the upper surface of the substrate and by receiving a reflected light of the first measurement laser beam; and

a separate-axis laser displacement meter configured to measure a vertical direction position of a second measurement irradiation point, which is distanced away from the processing irradiation point in a horizontal direction, by radiating, to the upper surface of the substrate, a second measurement laser beam having a different wavelength and a different path from the processing laser beam and the first measurement laser beam and by receiving a reflected light of the second measurement laser beam, and

wherein the controller selects either the coaxial laser displacement meter or the separate-axis laser displacement meter for each of the multiple dividing target lines before processing each dividing target line with the processing laser beam, and controls the vertical direction position of the condensing lens by using only one of selected meters while each of the multiple dividing target lines is processed with the processing laser beam,

wherein the separate-axis laser displacement meter obtains the vertical direction position of the processing irradiation point before a predetermined time period earlier than the coaxial laser displacement meter, and the controller controls the vertical direction position of the condensing lens based on a measurement result by the separate-axis laser displacement meter and monitors an intensity of the reflected light received by the coaxial laser displacement meter while a part of the multiple dividing target lines is processed, and

wherein when the monitored intensity of the reflected light is equal to or higher than a predetermined value, the controller performs a switchover of a laser displacement meter for controlling the vertical direction position of the condensing lens from the separate-axis laser displacement meter to the coaxial laser displacement meter, and controls a position of the condensing lens based on a measurement result by the coaxial laser displacement meter while a rest of the multiple dividing target lines is processed.