Lock structure for box

1. A lock structure for a box, comprising;

a first box member which has a retaining projection; and

a second box member which has an elastic retaining arm,

whererin the retaining arm has at its distal end a retaining portion for engagement with the retaining projection;

wherein the first box member has an arm restraint rib which restricts a releasing movement of the retaining arm in a condition in which the retaining arm and the retaining projection are engaged with each other;

wherein when the restraint rib is presented by the elastic retaining arm, the restraint rib is deformed by a pressing force of the elastic retaining arm; and

wherein a retaining distance of the retaining projection relative to the retaining portion of the elastic retaining arm is larger than a gap between the elastic retaining arm and the arm restraint rib in the engaged condition.

2. The lock structure according to claim 1 , wherein the retaining portion is formed on a first surface of the retaining arm; and

wherein the arm restraint rib is disposed so as to be opposed to a second surface of the retaining arm which is opposite to the first surface.

3. The lock structure according to claim 1 , wherein the arm restraint rib has a pair of arm restraint rib pieces;

wherein the pair of arm restraint rib pieces are formed so as to surround the second surface of the retaining arm; and

wherein a gap between the pair of arm restraint rib pieces is disposed at a position corresponding to a central position of the second surface of the retaining arm.