A watertight testing device for a pipe joint part, the watertight testing device conducting a watertight test on the pipe joint part in which a socket of one pipe receives an inserted spigot of another pipe, the pipe joint part being provided with a seal member between an inner surface of the socket and an outer surface of the spigot, the watertight testing device comprising: a cylindrical core fit into the pipe joint part; an annular first seal member for sealing between an outer surface of the core and an inner surface of the one pipe; an annular second seal member for sealing between the outer surface of the core and an 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 formed between the outer surface of the core and the inner surface of the one pipe; a second pressing member for pressing and compressing the second seal member into a second seal-member insertion space formed between the outer surface of the core and the inner surface of the other pipe; a moving device for moving the first and second pressing members (25, 26) in a pipe axial direction; and a test fluid feeder for feeding water into a test space formed between the outer surface of the core and the inner surface of the one and other pipe (2, 4) in a pipe diameter direction and between the first seal member and the second seal member in the pipe axial direction, characterized in that the annular first seal member is engaged with the first pressing member via a first engagement recess and a first engagement protrusion, and the annular second seal member is engaged with the second pressing member via a second engagement recess and a second engagement protrusion. the pipe joint part being provided with a seal member between an inner surface of the socket and an outer surface of the spigot, the watertight testing device comprising: a cylindrical core fit into the pipe joint part; an annular first seal member for sealing between an outer surface of the core and an inner surface of the one pipe; an annular second seal member for sealing between the outer surface of the core and an 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 formed between the outer surface of the core and the inner surface of the one pipe; a second pressing member for pressing and compressing the second seal member into a second seal-member insertion space formed between the outer surface of the core and the inner surface of the other pipe; a moving device for moving the first and second pressing members (25, 26) in a pipe axial direction; and a test fluid feeder for feeding water into a test space formed between the outer surface of the core and the inner surface of the one and other pipe (2, 4) in a pipe diameter direction and between the first seal member and the second seal member in the pipe axial direction, characterized in that the annular first seal member is engaged with the first pressing member via a first engagement recess and a first engagement protrusion, and the annular second seal member is engaged with the second pressing member via a second engagement recess and a second engagement protrusion. a cylindrical core fit into the pipe joint part; an annular first seal member for sealing between an outer surface of the core and an inner surface of the one pipe; an annular second seal member for sealing between the outer surface of the core and an 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 formed between the outer surface of the core and the inner surface of the one pipe; a second pressing member for pressing and compressing the second seal member into a second seal-member insertion space formed between the outer surface of the core and the inner surface of the other pipe; a moving device for moving the first and second pressing members (25, 26) in a pipe axial direction; and a test fluid feeder for feeding water into a test space formed between the outer surface of the core and the inner surface of the one and other pipe (2, 4) in a pipe diameter direction and between the first seal member and the second seal member in the pipe axial direction, characterized in that the annular first seal member is engaged with the first pressing member via a first engagement recess and a first engagement protrusion, and the annular second seal member is engaged with the second pressing member via a second engagement recess and a second engagement protrusion.