In an interlock system, locking of a second locking mechanism is enabled only when a first key holding part holds a first key and a control device is in a state in which a residual voltage of the control device is not discharged. Application of a power supply voltage to the control device is enabled by switching of a ground switch only when a second key holding part holds a second key. Removal of the second key from the second key holding part is enabled only when the ground switch is in the grounded position and the control device is in a grounded state. Removal of the first key from the first key holding part is enabled only when the control device is in a state in which residual voltage is discharged.