PURPOSE: To solve the problems existing in the conventional type watch case such as recurrence of dowil-sticking, position accuracy, stability in physical fixation, as well as to enhance thin thickness, miniaturization in size and water proofness by attaching and detecting back case on the case body utilizing elastic deformation of the packing.

CONSTITUTION: At the outer circumference 12a of the back case 12, packing 15 made of rubber, synthetic resin, etc., is fitted. Outer circumference 15a is closely attached to the inner side 14a of the case body 14, maintaining water proofness between the back case and the case body. Here, interference is given on the packing 15 to radial direction, and water proofness is secured by the elasticity of the packing between; a) the outer circumference 12a of the back case 12 and the inner circumference 15b of the packing, and b) the outer circumference 15a of the packing and the inner side 14a of the case body. On the upper part of the outer circumference of the packing 15, a protrusion 15a exists which sticks to and engage with the groove 14b provided at a part of the inner side of the case body 14. Elasticity of the packing 15 toward the radial direction pushes the protrusion 15a and the engagement of the back case 12 with the case body 14 is perfected. In disassembling the watch case, protrusion 15a moves toward the radial direction, and the disengagement is enabled in the same manner.