To provide a blocking-up apparatus for door clearance for preventing the door from being illicitly opened by making use of clearance between the door and a door frame.

This clearance blocking-up apparatus is installed in the door having a door lock for going in and out by a dead bolt; and is composed of a plate-like member for blocking up the clearance between the door and the door frame, and an installing member installed on a door frame side end surface of the door. The installing member is arranged on the reverse side of the plate-like member. A sensor is installed in any member side of the plate-like member or the installing member. The sensor is constituted for detecting a change in a relative position of the installing side plate-like member or the installing member, and the uninstalled side installing member or the plate-like member; and is provided with an alarm means for operating by a detecting signal from the sensor.