A resin molded parts lock mechanism for locking two resin molded parts (1, 2) together, the resin molded parts lock mechanism comprising: a lock hole formed in one resin molded part which is a housing; and a lock arm formed as a cantilever-shaped arm on the other resin molded part and adapted to be inserted into the lock hole from an entrance towards a deeper side thereof so as to be locked in the lock hole by making use of flexure thereof, and comprising: a first projecting portion and a second projecting portion, the first projecting portion being provided, of inner wall surfaces (11b, 11c), facing each other, of the lock hole which follow an inserting direction of the lock arm and face in flexing directions of the lock arm, on a first inner wall surface (11 b) in a position lying close to the entrance so as to project therefrom, the second projecting portion being provided on a second inner wall surface in a position lying deeper than the first projecting portion so as to project therefrom; and a permanent locking projection provided on a first side surface (21 a) of the lock arm in a position lying at a distal end in the inserting direction of the lock arm into the lock hole to face the second projecting portion so as to project therefrom, the permanent locking projection being adapted to ride over the second projecting portion as a result of the lock arm flexing in association with an inserting operation of the lock arm into the lock hole and then to be brought into engagement with the second projecting portion in a position where the permanent locking projection has ridden over the second projecting portion to thereby prevent the lock arm from being dislodged from the lock hole, characterized in that the other resin molded part is another housing, the lock hole and the lock arm have rectangular sections, and when the permanent locking projection is in engagement with the second projecting portion, the first projecting portion presses against a second side surface of the lock arm which lies opposite to the first side surface to thereby impart a reaction force directed towards the second projecting portion to the lock arm. a lock hole formed in one resin molded part which is a housing; and a lock arm formed as a cantilever-shaped arm on the other resin molded part and adapted to be inserted into the lock hole from an entrance towards a deeper side thereof so as to be locked in the lock hole by making use of flexure thereof, and comprising: a first projecting portion and a second projecting portion, the first projecting portion being provided, of inner wall surfaces (11b, 11c), facing each other, of the lock hole which follow an inserting direction of the lock arm and face in flexing directions of the lock arm, on a first inner wall surface (11 b) in a position lying close to the entrance so as to project therefrom, the second projecting portion being provided on a second inner wall surface in a position lying deeper than the first projecting portion so as to project therefrom; and a permanent locking projection provided on a first side surface (21 a) of the lock arm in a position lying at a distal end in the inserting direction of the lock arm into the lock hole to face the second projecting portion so as to project therefrom, the permanent locking projection being adapted to ride over the second projecting portion as a result of the lock arm flexing in association with an inserting operation of the lock arm into the lock hole and then to be brought into engagement with the second projecting portion in a position where the permanent locking projection has ridden over the second projecting portion to thereby prevent the lock arm from being dislodged from the lock hole, characterized in that the other resin molded part is another housing, the lock hole and the lock arm have rectangular sections, and when the permanent locking projection is in engagement with the second projecting portion, the first projecting portion presses against a second side surface of the lock arm which lies opposite to the first side surface to thereby impart a reaction force directed towards the second projecting portion to the lock arm.