In an inner fin 2 inserted into a tube 4 and exchanging heat, a plate material is composed of a top portion 10 kept into contact with an upper plate portion of the tube, a bottom portion 12 kept intocontact with a lower plate portion, and a wall plate portion 14 partitioning them. Flow channels having concave cross-sections and flow channels having reverse concave cross-sections are alternately repeatedly formed by a pair of opposed wall plate portions as a flow channel of a gas, the wall plate portions of the flow channels are bent zigzag right and left to form the shape on which projectingportions 20 and recessed portions 22 are alternately repeatedly formed, and a chevron-shaped portion 24 swollen in a direction of a wall plate portion opposed to the wall plate portion, and composed of an upward inclination 28 from a base portion to a top portion and a downward inclination portion 29 from the top portion to an adjacent base portion, is formed on the recessed portion 22 of the wallplate portion.