PURPOSE: To prevent damage to a pyroelectric element and make it less likely to be affected by a change of circumferential temperature by disposing a heat beam generating member to generate a heat beam corresponding to the temperature of a secondary battery and the pyroelectric element to receive the heat beam and detect a temperature change to face each other having a space between them.

CONSTITUTION: A substrate 5 where a pyroelectric element 3 and an FET 4 are mounted is contained in a case 2 to face the pyroelectric element 3 with the top surface of the case 2 having a space 7 between them. The top surface of the case 2 in contact with a secondary battery 10 generates a heat beam corresponding to the temperature of the secondary battery 10, and this heat beam gets into the pyroelectric element 3 to be converted into an electric signal to be outputted to the external. direct transmission of an external impact to the pyroelectric element 3 is thus prevented by the space 7, and through the pyroelectric element 3 catches the heat beam from the heat generating member facing it at a high efficiency, it catches a heat beam from parts not facing it at a low efficiency. Breakage of the pyroelectric element 3 by the external impact can thus be prevented, and the battery becomes less likely to be affected by a circumferential temperature change.