Lock device and electric power steering system

A steering lock device includes: a lock pin (26) that is engageable with an engagement portion, a compression spring (28) that urges the lock pin in a direction away from the engagement portion; and an actuator (30) that moves the lock pin to engage the engagement portion against the spring force by applying current to the coil (54). When current application is stopped, the lock pin is maintained at the lock end by the actuator plunger (51) formed of a magnet with one pole pair.