An exhaust gas treatment device for a diesel engine is capable of smoothly starting the generation of a combustible gas and is provided with a small combustible gas generator. A core material is fitted in the center portion of an annular wall to form an air-fuel mixing chamber between the inner peripheral surface of the annular wall and the outer peripheral surface of the core material. An air-fuel mixture gas in the air-fuel mixing chamber is adapted to be supplied from the end of the air-fuel mixing chamber to a combustible gas generating catalyst to a portion near the center thereof. A heater is used as the core material, the heat dissipating outer peripheral surface of the heater is exposed to the air-fuel mixing chamber. Heat is dissipated directly from the heat dissipating outer peripheral surface of the heater to the air-fuel mixing chamber when starting the generation of the combustible gas.