A locking member (30, 530) configured to be attached to a first connector (10, 510), the locking member (30, 530) comprising: a regulation portion (32, 531) configured to regulate an enlargement of a cylindrical fitting portion (11a, 511a) of a first contact (11, 511) of the first connector (10, 510), the cylindrical fitting portion (11a, 511a) being electrically connected and attached to a contact of a mating connector (100, 600), wherein an outer diameter of the cylindrical fitting portion (11a, 511a) is configured to be temporarily enlarged when the cylindrical fitting portion (11a, 511 a) is being attached to the contact of the mating connector (100, 600), and wherein the regulation portion (32, 531) is configured to prohibit an enlargement of the cylindrical fitting portion (11a, 511a) associated with removing the cylindrical fitting portion (11a, 511a) from the contact of the mating connector (100, 600), characterized in that the regulation portion (32, 531) includes a pair of arm portions (37, 38, 531) configured to regulate the enlargement of the cylindrical fitting portion (11a, 511a) by sandwiching the cylindrical fitting portion (11a, 511a) from an outside radial direction; wherein the locking member (30, 530) further includes a guide portion (33, 532) and the guide portion (33, 532) includes a sliding portion (39, 536), the sliding portion (39, 536) being configured to slide along a rail portion (11e, 511c) of the first connector to guide the regulation portion (32, 531) as it moves in an axial direction intersecting the direction of engagement of the cylindrical fitting portion (11a, 511a) between a regulation position associated with regulating the enlargement of the cylindrical fitting portion (11a, 511a) and a standby position in which the enlargement of the cylindrical fitting portion (11a, 511a) is not prohibited by the regulation portion (32, 531); wherein a gap between the pair of arm portions (37, 38, 531) is narrower than the outer diameter of the cylindrical fitting portion (11a, 511a) when it is enlarged; wherein the guide portion (33, 532) is configured to guide the movement of the regulation portion (32, 531) in a locking direction intersecting a direction of engagement of the cylindrical fitting portion (11a, 511a); wherein the locking direction in which the guide portion (33, 532) guides the movement of the regulation portion (32, 531) is in the axial direction. a regulation portion (32, 531) configured to regulate an enlargement of a cylindrical fitting portion (11a, 511a) of a first contact (11, 511) of the first connector (10, 510), the cylindrical fitting portion (11a, 511a) being electrically connected and attached to a contact of a mating connector (100, 600), wherein an outer diameter of the cylindrical fitting portion (11a, 511a) is configured to be temporarily enlarged when the cylindrical fitting portion (11a, 511 a) is being attached to the contact of the mating connector (100, 600), and wherein the regulation portion (32, 531) is configured to prohibit an enlargement of the cylindrical fitting portion (11a, 511a) associated with removing the cylindrical fitting portion (11a, 511a) from the contact of the mating connector (100, 600), characterized in that the regulation portion (32, 531) includes a pair of arm portions (37, 38, 531) configured to regulate the enlargement of the cylindrical fitting portion (11a, 511a) by sandwiching the cylindrical fitting portion (11a, 511a) from an outside radial direction; wherein the locking member (30, 530) further includes a guide portion (33, 532) and the guide portion (33, 532) includes a sliding portion (39, 536), the sliding portion (39, 536) being configured to slide along a rail portion (11e, 511c) of the first connector to guide the regulation portion (32, 531) as it moves in an axial direction intersecting the direction of engagement of the cylindrical fitting portion (11a, 511a) between a regulation position associated with regulating the enlargement of the cylindrical fitting portion (11a, 511a) and a standby position in which the enlargement of the cylindrical fitting portion (11a, 511a) is not prohibited by the regulation portion (32, 531); wherein a gap between the pair of arm portions (37, 38, 531) is narrower than the outer diameter of the cylindrical fitting portion (11a, 511a) when it is enlarged; wherein the guide portion (33, 532) is configured to guide the movement of the regulation portion (32, 531) in a locking direction intersecting a direction of engagement of the cylindrical fitting portion (11a, 511a); wherein the locking direction in which the guide portion (33, 532) guides the movement of the regulation portion (32, 531) is in the axial direction.