A signal processing device (19) executes a phase connection process step of performing a phase connection process on a position in a space and a phase value at each of a plurality of times, a first correction step of performing outlier correction of a phase value for each position in the space in a predetermined direction of the space at a predetermined time among the plurality of times based on a result of the phase connection process, and a second correction step of correcting a phase value at a time other than the predetermined time for each of correction target positions in the first correction step among positions in the space.