A water tube WT of a heat exchanger B includes a helical tube body 5 which includes a plurality of loops 50 arranged in the axial direction. At least one of the loops 50 includes an inclined tube portion which is inclined with respect to the axial direction and a non-inclined tube portion extending perpendicularly to the axial direction. The inclined tube portion is provided at each of opposite end regions s2a and s2b in a width direction crossing the axial direction, whereas the non-inclined tube portion is provided at an intermediate region S1 in the width direction. With this structure, the pitch p2 of the loops 50 is reduced. Therefore, by increasing the number of loops 50, high heat exchange efficiency is achieved without considerably increasing the size of the helical tube body 5.