PURPOSE: To realize an infellible slip detection by a detective element by a method wherein a mixture gas blow outlet of a mixing chamber is formed smaller than the external size of a main catalyst layer and opposed substantially to a central part of the main catalyst layer, and the detective element is provided at a part opposing the mixture gas blow outlet.

CONSTITUTION: A mixture gas blow outlet 6 is designed to be smaller than the external size of a main catalyst layer 8 and opposed substantially to a central part of the main catalyst layer 8. A heat sensitive element 12 to detect temperature is provided between the main catalyst layer 8 and a subsidiary catalyst layer 9, and guided by a heat sensitive element guide pipe 13 to a position which opposes to a mixture gas blow outlet 6 and is substantially central in a radial direction of the main and the subsidary catalyst layers 8 and 9. Thereby, flow velocity of premixed gas of fuel and air to be supplied to the main catalyst layer part becomes maximum at the cental part in the radial direction of the main catalyst layer 8, and thereby, a position where a first slip occurs can be limited where the premixed mixture flow velocity is maximum, that is, the central part in the radial direction of the main catalyst layer 8.