To prevent formation of a gap causing no communication of inside section with the outside section in a case fitting section even if an opening force Fz is applied to cases 1 and 2 fitted to each other.

A type C joint member 21 and a locking protrusion part 10 are brought into contact with each other due to case fitting, as a result, the joint member 21 turns R in the vertical direction with a hinge part 20a as a fulcrum and whose locking claws 21a and 21b are hooked to locking holes 11a and 11b respectively for locking both cases 1 and 2. After that, when external force Fz is applied in the direction to open joint end faces 1a and 2a, the joint member 21 starts to turn R in the direction opposite to the fitting direction due to the external force Fz taking the hinge section 20a as a fulcrum. Force Fy is generated to pull the locking projection section 10 in the direction outside of the cases from the upper side locking claw 21a through the upper side locking hole 11a, as a result, the projected groove sides face 12a and the recessed groove inside face 13a of a fitting recessed and projected grooves 12 and 13 are pressed against each other by the force Fy to pull the one side case outward, resulting in keeping the inside of the cases airtight.