To enhance the certainty of holding an arm etc.

A brake device includes a brake disk, a plurality of armatures, a moving mechanism, a plurality of locked parts, and a plurality of locking parts. The brake disk rotates as a shaft that an electric rotary machine has rotates. The plurality of armatures are arranged opposite the brake disk. The moving mechanism moves the plurality of armatures relatively to the brake disk. The plurality of locked parts area provided on an armature-side plate surface of the brake disk at predetermined intervals. The plurality of locking parts are provided on brake disk-side plate surfaces of the plurality of armatures at such intervals that the locking parts can engage the locked parts. A locking part formed on at least one of the plurality of armatures is different in phase from the locking parts formed at the other armatures.