PURPOSE: To obtain a heater having a large capacity, high performance and being inexpensive in the heater to be used for a heat source of room heater/ cooler in an air conditioner field.

CONSTITUTION: This heater has a plurality of heating passages 1 provided in parallel, heat absorption fins 2 provided on the outer surfaces of the passages 1, a burner 3 for heating the passages 1 and the fins 2 and a casing 4 for guiding combustion gas generated by the burner 3 to the passages 1 having the fins 2, and comprises a straightening plate 6 provided at a downstream side of the passages 1 oppositely to the burner 3 to block between the passages 1 and having a slit 7 for opening the rear of the passages 1. A flowing speed of the gas flowing between the passages 1 is made uniform. Even if the flow rate of the gas is increased, characteristics of high heat exchanging efficiency are obtained.