To provide a biological treatment apparatus capable of thermally and biologically teating sludge successively while performing exhaustion of gas without clogging an air taking-out passage to reduce a sludge forming amt.

A biological treatment apparatus has a heat treatment tank 1 receiving bioligical sludge to heat-treat the same, a biological treatment tank 3 receiving the heat-treated sludge of the heat treatment tank 1 to biologically treat the same and a gas-liquid separation tank 2 receiving gas and an accompanied liquid from the gaseous phase part of the heat treatment tank 1 to separate the same into air and a liquid and supplying both of them to the biological treatment tank 3.