There are provided an ultrasonic waveform data generation unit (123) that generates ultrasonic waveform data representing amplitudes of reflected ultrasonic beams received by a reception unit (132) in time series for each of ultrasonic elements (111) forming reception ultrasonic elements (114), an ultrasonic waveform data processing unit (124) that uses a plurality of origin time adjustment patterns, in which a plurality of relative positional relationships between a phased array probe (110) and a welded portion (210) are set, and performs processing to synthesize pieces of the ultrasonic waveform data each having the adjusted origin time to generate synthesized ultrasonic waveform data according to each of the origin time adjustment patterns, and a defect evaluation unit (125) that evaluates whether or not a defect (211) is present in the welded portion (210) based on the synthesized ultrasonic waveform data.