To provide a differential temperature meter for detecting a temperature difference between indoor and outdoor with no use of an arithmetic unit.

An external combustion engine E which heats/cools the gas in a cylinder 2 to obtain motion through expansion/contraction of the gas is provided on either one side of a partition 1 that separates indoor from outdoor. A heat conduction member 13 is provided to the heating part or the cooing part of the external combustion engine E. The heat conduction member 13 is extended from one side of the partition 1 to the other side, thereby representing a temperature difference between indoor and outdoor by the motion.