To provide a security device in which no dead angle exists in the detection of an intruder, a detection part for the purpose is not revealed and which can work at inexpensive cost.

A surface heating element for a floor heater is used as a structural element for a two-dimensional detection means. A metallic sheet 1(3) is spread over a back heat insulation material 1(2) of the surface heating element 1(1) spread over the back surface of a floor of a closed floor heating space in a building and a high frequency signal is added to the surface heating element 1(1) and the metallic sheet 1(3). The high frequency signal indicating vacant state in which the floor heating space is not used, is stored in advance and the high frequency signal is added to the surface heating element 1(1) and the metallic sheet 1(3) in the vacant state. When the high frequency signal indicates a state which is different from the vacant state, this is responded and an alarm is generated. According to this structure, whenever an intruder steps on the floor, intrusion is detected.