A watertight testing device includes a cylindrical core fit into a pipe joint part, a first seal member for sealing between the outer surface of the core and the inner surface of one pipe, a second seal member for sealing between the outer surface of the core and the inner surface of the other pipe, a first pressing member for pressing and compressing the first seal member into a first seal-member insertion space, a second pressing member for pressing and compressing the second seal member into a second seal-member insertion space, a moving device for moving the first and second pressing members in a pipe axial direction, and a test fluid feeder for feeding a test fluid into a test space.