A defect detection device configured to detect a defect present in a welded portion formed along a pipe longitudinal direction of a welded steel pipe, the defect detection device comprising: a phased array probe that is installed outside an outer surface of the welded steel pipe and includes a plurality of ultrasonic elements arranged therein; a transmission means configured to transmit ultrasonic beams toward the welded portion through the outer surface of the welded steel pipe from the ultrasonic elements; a reception means configured to receive reflected ultrasonic beams being the ultrasonic beams reflected on the welded steel pipe including the welded portion by each of the ultrasonic elements; an ultrasonic waveform data generation means configured to generate ultrasonic waveform data representing amplitudes of the reflected ultrasonic beams received by the reception means in time series for each of the ultrasonic elements; an origin time adjustment pattern information acquisition means configured to acquire origin time adjustment pattern information including a plurality of origin time adjustment patterns for adjusting an origin time of ultrasonic waveform data for each of the ultrasonic elements, wherein the plurality of the origin time adjustment patterns correspond to optimum reception delay patterns prepared responsive to respective deviation angles in a circumferential direction of a calibration pipe, wherein the calibration pipe comprises an artificial defect formed in a second welded portion equivalent to the welded portion, wherein relative to the phased array probe, a reference position of the second welded portion is set as a first position, and wherein the deviation angle is the angle corresponding to a position where the position of the second welded portion is deviated from the first position in the circumferential direction of the calibration pipe by the angle, wherein the origin time is a time indicating a starting point of each received waveform when synthesizing received waveforms and is a time when a predetermined time has elapsed from a starting point of transmission or reception of the ultrasonic beams, an ultrasonic waveform data processing means configured to shift the origin time of each of the ultrasonic waveform data generated for each of the ultrasonic elements to adjust the origin time so as to be the same defect echo appearance time by using the plurality of the origin time adjustment patterns, and perform processing to synthesize pieces of the ultrasonic waveform data each having the adjusted origin time to generate synthesized ultrasonic waveform data; and a defect evaluation means configured to evaluate whether or not the defect is present in the welded portion based on the synthesized ultrasonic waveform data. a phased array probe that is installed outside an outer surface of the welded steel pipe and includes a plurality of ultrasonic elements arranged therein; a transmission means configured to transmit ultrasonic beams toward the welded portion through the outer surface of the welded steel pipe from the ultrasonic elements; a reception means configured to receive reflected ultrasonic beams being the ultrasonic beams reflected on the welded steel pipe including the welded portion by each of the ultrasonic elements; an ultrasonic waveform data generation means configured to generate ultrasonic waveform data representing amplitudes of the reflected ultrasonic beams received by the reception means in time series for each of the ultrasonic elements; an origin time adjustment pattern information acquisition means configured to acquire origin time adjustment pattern information including a plurality of origin time adjustment patterns for adjusting an origin time of ultrasonic waveform data for each of the ultrasonic elements, wherein the plurality of the origin time adjustment patterns correspond to optimum reception delay patterns prepared responsive to respective deviation angles in a circumferential direction of a calibration pipe, wherein the calibration pipe comprises an artificial defect formed in a second welded portion equivalent to the welded portion, wherein relative to the phased array probe, a reference position of the second welded portion is set as a first position, and wherein the deviation angle is the angle corresponding to a position where the position of the second welded portion is deviated from the first position in the circumferential direction of the calibration pipe by the angle, wherein the origin time is a time indicating a starting point of each received waveform when synthesizing received waveforms and is a time when a predetermined time has elapsed from a starting point of transmission or reception of the ultrasonic beams, an ultrasonic waveform data processing means configured to shift the origin time of each of the ultrasonic waveform data generated for each of the ultrasonic elements to adjust the origin time so as to be the same defect echo appearance time by using the plurality of the origin time adjustment patterns, and perform processing to synthesize pieces of the ultrasonic waveform data each having the adjusted origin time to generate synthesized ultrasonic waveform data; and a defect evaluation means configured to evaluate whether or not the defect is present in the welded portion based on the synthesized ultrasonic waveform data.