Abstract: It is sought to reduce combustion rise time at the start of driving, increase the combustion efficiency per unit heat exchanger volume and reduce the heater size. A ring-like fuel evaporating medium 4 is provided on an end wall of a combustion chamber formed in a combustion cylinder 1. The end wall has a combustion air lead holes formed, for forming a vortex flow, in its portion surrounded by the fuel evaporating medium. Fuel is supplied to a space in the vicinity of an upper part of the fuel evaporating medium. A combustion gas passage is formed by proving a first combustion gas collecting zone downstream a second combustion chamber adjacent to the first combustion chamber. A second combustion gas collecting zone 10 is provided around the first and second combustion chambers. The first and second combustion gas collecting zones 9 and 10 are communicated with each other by a plurality of connecting pipes. Combustion gas is exhausted from the second gas collecting zone 10.