The steering lock device has: a lock part which is held in a lock part accommodation recess formed on a frame and which can move between the locked position and the unlocked position; and a plug which can be operated to rotate from the outside of the frame and which causes the lock part to move to the unlocked position against a reaction force of a compression spring. One end of the lock part is biased toward the side of the locked position by the compression spring supported on a spring seat formed on the ceiling part of the lock part accommodation recess. The spring seat is formed so as to project from the ceiling surface of the lock part accommodation recess toward the opposing surface.