To enable high-sensitivity measurement without using a chopper, by inputting an initial value into a memory circuit of a pyroelectric infrared sensor, and by adding to the initial value, a difference signal based on infrared energy inputted at an optional time thereafter, to obtain a continuous signal.

Infrared detecting elements 3 are arranged two-dimensionally, and plural MOSFET type switches 4 and plural memory circuits 5 corresponding to the infrared detecting elements 3 are installed, inside a case 2 of a pyroelectric infrared sensor 1, and a shutter 6 is installed on the front of the case 2. With this formation, for example, one-shot shut of the shutter 6 is executed at the time of power source input, then an initial value is inputted. The initial value is memorized in a memory circuit 5 installed corresponding to each infrared detecting element 3 of the pyroelectric infrared sensor 1. After a proper time elapses from the point of time of initial value input, a difference value is inputted, and the difference value is operated in addition to the initial value memorized in the memory circuit 5, to thereby obtain a continuous signal.