An ultrasonic testing device includes: a liquid tank that stores the liquid into which the differential casing is to be immersed; a workpiece holder that holds the differential casing and immerses the differential casing into the liquid in the liquid tank; an ultrasonic probe that conducts an ultrasonic testing of the welded part of the differential casing by irradiating the ultrasonic wave to the welded part and receiving a reflected wave; an image processing module that generates an ultrasonic image based on the reflected wave received by the ultrasonic probe; and a replacer that replaces gas inside an air gap formed at a weld toe of the welded part with test liquid. The replacer replaces the gas inside the air gap by the test liquid before the ultrasonic probe irradiates the ultrasonic wave.