a first bulkhead;

a second bulkhead; and

a plurality of flow path walls which divide a space formed between the first bulkhead and the second bulkhead into a plurality of first flow paths, wherein

the first bulkhead and the second bulkhead separate the plurality of first flow paths from a second flow path through which a second fluid different from a first fluid flowing through the plurality of first flow paths flows,

a plurality of wall surfaces are formed on the plurality of flow path walls,

each of the plurality of wall surfaces conforms to a sine curve at different positions,

two adjacent flow path walls among a plurality of sinusoidal flow path walls arranged in an amplitude direction of the sine curve are sinusoidal flow path walls having a phase range of θ0 (=0°)<θ1<θ2<90°<θ3<θ4<180°<θ5<θ6<270°<θ7<θ8<θ0 (=360°) as one period when a phase overlapping an inflection point of a sine curve of one flow path wall is θ0 (=0°),

in the one flow path wall, a main flow path wall element is formed in a portion overlapping a range of a phase θ of θ1≤θ<θ3 and θ6≤θ<θ8 by forming a notch which does not have a flow path wall in a portion overlapping a range of a phase θ of θ0≤θ<θ1 and θ8≤θ<θ0 and forming an in-element notch which does not have a flow path wall formed in a portion overlapping a range of a phase θ of θ3≤θ<θ6,

in an other flow path wall, a main flow path wall element is formed in a portion overlapping a range of a phase θ of θ2≤θ<θ4 and θ5≤θ<θ7 by forming a notch which does not have a flow path wall formed in a portion overlapping a range of a phase θ of θ0≤θ<θ2 and θ7≤θ<θ0 and forming an in-element notch which does not have a flow path wall formed in a portion overlapping a range of a phase θ of θ4≤θ<θ5, and

one flow path of the plurality of first flow paths is connected, via the notches and the in-element notches formed on the one flow path wall and the other flow path wall of the plurality of flow path walls, to two adjacent flow paths arranged respectively on both sides of the one flow path of the plurality of first flow paths.