To provide a septic tank capable of decreasing running cost.

This septic tank is buried in the ground and has a septic chamber 1. The chamber 1 consists of a housing 2 which forms its external walls, and a manhole 3 which closes an opening 2a of the side of the housing 2 which is exposed to the air. Solar batteries 12 are equipped on the outer face of the manhole 3. The manhole 3 is equipped with Peltier elements 13, and on both ends of the Peltier elements heat transfer fins 13a, 13b are attached in contact thereto. The solar battery 12 is connected to a battery 14, and the battery 14 is connected to the Peltier elements 13 via a switch 15, by which polarity of the Peltier elements are changable.