The present invention provides an oil seal cap that can be attached to a part to be closed without any laborious and time-consuming step and can achieve an efficient oil passage operation such as an oil changing operation. An oil seal cap 30 includes: a body part 31 having a first hole 31a passing therethrough; a port part 32 provided on the body part 31 inside an outer circumferential part of the body part 31 when viewed in the passing-through direction of the first hole 31a, the port part 32 having a second hole 32a passing therethrough and extending in the same direction as the passing-through direction of the first hole 31a; and a plug member 33 configured to block the second hole 32a, the plug member 33 being removably mounted on the port part 32. The second hole 32a is opened, through the first hole 31a, toward a side opposed to a side where the port part 32 is provided on the body part 31; and a surface of the body part 32 is provided with a locking part 38a projecting with respect to the surface.