To automatically open a display part side body to a specified angle open position by a push button pressing operation in the state of the display part side body folded on an operation part side body while securely holding the display part side body at the folded position.

A lock mechanism W locking the rotation of the other body B2 when the other body B2 is rotated to a position where the other body is double-folded on one body B1 is installed in one hinge part C of a hinge part provided on the left end and right end of an abutted part between the display part b side body B2 and the operation part a side body B1. A push button P to release the lock by pressing is installed therein. A spring material k having a spring pressure to rotate the other body B2 to a specified open angle in an opening direction in the state of the other body B2 double-folded on one body B1 is installed in the other hinge part C'.