An exhaust treatment device of a diesel engine is provided in which combustible gas is burned with oxygen in exhaust, combustion heat increases a temperature of the exhaust, and heat of the exhaust can burn and remove PM accumulating in a DPF. In order to cause a heater for radiating heat at a start of generation of the combustible gas to enter a catalyst inlet portion, and fit a liquid fuel retaining member over a periphery of the heater, a guide plate is provided to a lower face of the liquid fuel retaining member so that the air-fuel mixture moving down in the liquid fuel retaining member flows along an upper face of the guide plate out to a periphery of the guide plate.