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