a first signal processor that receives input of a plurality of video signals, each including a plurality of packets, and multiplexes the plurality of video signals to output a multiplexed video signal via a single output system; and

a second signal processor that receives input of the multiplexed video signal and demultiplexes the multiplexed video signal to output the plurality of video signals, wherein:

changing, by an instruction from a user of the video signal processing device via an input receiver, a channel that carries any video signal out of the plurality of video signals and is to be used as the input of the first signal processor, causes the first signal processor to transmit a notification for starting detection processing for detecting deficient data whose size is less than a size of one packet with respect to the video signal of which channel is changed, to the second signal processor,

the second signal processor starts the detection processing upon receiving the notification, and

when the deficient data is detected in the video signal of which channel is changed, the second signal processor performs one of (i) processing for discarding the deficient data detected and (ii) processing for adding dummy data to the deficient data detected to make a size of the deficient data equal to a size of an integral multiple of one packet, the integral multiple being a multiple of an integer of 1 or greater.