To provide an intrusion detecting device of a type in which a battery for supplying an electric power to both of the main unit and the transmitter is provided in a unit holder for holding the main unit, including the detecting element, to thereby increase the overall electric power capacity needed in the intrusion detecting device as a whole and, also, replacement of those batteries can be accomplished easily, the intrusion detecting device includes a main unit (1) including a detecting element (30), a battery (Ba) detachably mounted on the main unit (1) for supplying an electric power to the main unit (1), and a power supply line (18) fitted to the main unit (1) for supplying an electric power to a wireless transmitter (11) for transmitting wireless an output signal of the main unit (1).