PURPOSE: To lessen the number of the manufacturing processes, easily detect that battery changing work is started, make assembly work easy, and reduce the cost.

CONSTITUTION: A pair of springs 25 for a MP switch are installed on the lower face of a clock module 22 and a contact plate 26 is installed at the upper face position of a module pressing member 23 corresponding to these springs. Whether the module pressing member 23 is inserted in and attached to a clock case 21 or not, in other words whether the springs 25 for a MP switch are electrically conducted through the contact plate 26 or not, is detected. A switch is so composed as to have a data retention controlling circuit which can interrupt reading-out and writing-in of involatile memory in the clock module at conveniently dividual point in the case of the electrically not conducted state (the module pressing member 23 is not attained to the clock case 21). Consequently, whether battery changing work is started or not, can be detected with a simple structure and the number of the parts can be lessened and the manufacturing processes can be simplified.