To provide a combustion device formed that when a kind of gas used is reciprocally converted to low calorie gas and high calorie gas, a combustion system is easily varied to a Bunsen system and a Semibunsen system or a thick and thin fuel combustion system, and the thick and thin fuel combustion system and the Bunsen and semibunsen system can be both established.

A combustion device is provided with a burner unit U, which juxtaposes a number of thick and thin combustion burners 1 where a thin combustion burner part 2 to deliver thin gas, low in fuel gas concentration, through a main flame hole 23 in an upper end burn the thin gas as a main flame M1, and a thick combustion burner part 3 to deliver thick gas, high in fuel gas concentration, through auxiliary burner ports 33 formed adjacently to two outsides of the main flame hole and burn it as a retention flame 2 are combined together. When low calorie gas or high calorie gas is used as fuel gas, thick gas being high in fuel gas concentration is respectively fed to the thin combustion burner part and the thick combustion burner part to effect thick combustion.