PURPOSE: To provide a relay unit in which initial resistance can be kept at a low level while reducing power consumption significantly during heat balance.

CONSTITUTION: A first positive characteristic thermistor has one end connected with a terminal 4 for external connection and the other end connected with one end of a second positive characteristic thermistor 2 having the other end connected with another terminal 5 for external connection. A heat response switch 3 is thermally coupled with the first and second positive characteristic thermistors 1, 2 and electrically connected in parallel with the second positive characteristic thermistor 2. The heat response switch 3 is normally closed and opened in response to heat being imparted from the first or second positive characteristic thermistor 1 or 2.