An oil seal cap comprising: a body part (31, 313, 314, 316) having a first hole passing therethrough; a port part (32, 322, 325) provided on the body part (31, 313, 314, 316) inside an outer circumferential part of the body part (31, 313, 314, 316) when viewed in the passing-through direction of the first hole, the port part (32, 322, 325) having a second hole passing therethrough and extending in the same direction as the passing-through direction of the first hole; and a plug member configured to block the second hole, the plug member being removably mounted on the port part (32, 322, 325); wherein: the body part (31, 313, 314, 316) is configured to be attached to a through-hole as a part to be closed by the outer circumferential part of the body part (31, 313, 314, 316) being in tight contact with the through-hole; the second hole is opened, through the first hole, toward a side opposed to a side where the port part (32, 322, 325) is provided on the body part (31, 313, 314, 316); a surface of at least one of the body part (31, 313, 314, 316) and the port part (32, 322, 325) is provided with a locking part (38a, 38b, 38c, 38d) projecting or recessed with respect to the surface; and the locking part (38a, 38b, 38c, 38d) is formed so as to be locked by a tool. a body part (31, 313, 314, 316) having a first hole passing therethrough; a port part (32, 322, 325) provided on the body part (31, 313, 314, 316) inside an outer circumferential part of the body part (31, 313, 314, 316) when viewed in the passing-through direction of the first hole, the port part (32, 322, 325) having a second hole passing therethrough and extending in the same direction as the passing-through direction of the first hole; and a plug member configured to block the second hole, the plug member being removably mounted on the port part (32, 322, 325); wherein: the body part (31, 313, 314, 316) is configured to be attached to a through-hole as a part to be closed by the outer circumferential part of the body part (31, 313, 314, 316) being in tight contact with the through-hole; the second hole is opened, through the first hole, toward a side opposed to a side where the port part (32, 322, 325) is provided on the body part (31, 313, 314, 316); a surface of at least one of the body part (31, 313, 314, 316) and the port part (32, 322, 325) is provided with a locking part (38a, 38b, 38c, 38d) projecting or recessed with respect to the surface; and the locking part (38a, 38b, 38c, 38d) is formed so as to be locked by a tool. the body part (31, 313, 314, 316) is configured to be attached to a through-hole as a part to be closed by the outer circumferential part of the body part (31, 313, 314, 316) being in tight contact with the through-hole; the second hole is opened, through the first hole, toward a side opposed to a side where the port part (32, 322, 325) is provided on the body part (31, 313, 314, 316); a surface of at least one of the body part (31, 313, 314, 316) and the port part (32, 322, 325) is provided with a locking part (38a, 38b, 38c, 38d) projecting or recessed with respect to the surface; and the locking part (38a, 38b, 38c, 38d) is formed so as to be locked by a tool.