To provide a safety lock that achieves the largest possible magnetic retaining force.

This invention relates to a lock device (10) equipped with an electromagnet (13) and a yoke plate (23) that closes a magnetic circuit of the electromagnet (13). In this lock device, the electromagnet (13) is installed by means of an installation means (31, 33) in the way that it can rotate around a point on the central axis line and has a tensile strength in the longitudinal direction of this line. The central axis line extends in the direction perpendicular to the surface where the electromagnet (13) is in contact with the yoke plate (23), and through the retaining force of the electromagnet (13) with its circuit closed. Thus, the force to separate the yoke plate (23) from the polar surface (15) in reaction to the magnetism always acts coaxially with the magnetism, and this hinders the yoke plate (23) from being separated from the polar surface (15) on one side.