PURPOSE: To reduce a generation of nitride by a method wherein unburnt gas is reduced by arranging oxide catalyst of film form or particle form at a circumferential wall or around a circumference of flame forming hole.

CONSTITUTION: An outer cylinder 4 and an inner cylinder 5 for use in guiding the lifting or descending of a combustion wick 3 are arranged at a main body of a combustion unit, and mounting plates 7 and 7 on which a burner 6 (a flame cylinder) at the upper end thereof by bending the mounting plates in an opposite direction to each other. The burner 6 is composed of an inner flame cylinder 9 and an outer flame cylinder 8, and oxidation catalyst is arranged at an entire part of the hole circumferential wall 1a or a part of 1/3W2/5 of an entire height from the lower end thereof where a temperature of surrounding atmosphere is low. When ignited, although the flame is burnt while being adhered to the wick 3 for a while, as a temperature of the wick 3 is increased, the flame is floated up from the wick 3 and then a primary combustion is carried out at a space enclosed by the inner and outer flame cylinders 9 and 8. The secondary air is taken from the flame forming holes 1 after exiting the burner 6 and then a secondary combustion is carried out.