To prevent nozzle clogging which is caused by the fact that a liquid component of waste liquid evaporates inside a liquid fuel jetting nozzle 5 and impurities contained in the waste liquid coagulate and stick to the nozzle, relating to a combustion device which performs a multi-fuel combustion operation with gas fuel and liquid fuel as well as a mono-fuel combustion operation only with gas fuel.

The combustion device performs a multi-fuel combustion operation with gas fuel and liquid fuel as well as a mono-fuel combustion operation only with gas fuel. The liquid fuel is jetted from a liquid fuel jetting nozzle 5 provided near the tip of the combustion device. The liquid fuel is supplied to the liquid fuel jetting nozzle 5 through a liquid fuel supply pipe 1. The liquid fuel supply pipe 1 is provided with a shrinkable pipe 9 which extends when applied with a pressure when supplying liquid fuel and shrinks when the pressure disappears when stopping supply of the liquid fuel. The liquid fuel jetting nozzle 5 shifts to and fro along the combustion device depending on the presence of liquid fuel supply.