PURPOSE: To enable specifying of a leaky heat transfer pipe by a method wherein when the leaky heat transfer pipe can not be specified visibly, sheets for detection are inserted from two directions symmetrical with respect to a piping row in a manner to contain the heat transfer pipe to be surveyed, and outflow water drops are confirmed.

CONSTITUTION: When a heat transfer pipe 8d leaks, water drops are detected at the outer peripheral group of a pipe group. A specified number of sheets 20a for detection having size large enough to allow carriage of them through the manhole of an exhaust heat recovery heat exchanger from the upper part or the lower part of a pipe group is inserted along the alignment direction of the heat transfer pipe in the vicinity of a place where the water drops are detected. When the sheets 20a for detection are inserted in a manner to nip the heat transfer pipe 8d therebetween, water drops flowing out from the heat transfer pipe 8d flow along the sheets 20a for detection flows to a lower end. Water drops 21 are conformed, and after a row 22 of the heat transfer pipe is distinguished by means of a discriminating mark, the sheets 20a for detection are pulled off, and is inserted along alignment of the heat transfer pipe symmetrically to a preceding direction. This method enables the heat transfer pipe 8d being an intersection between two rows to be specified as a heat transfer, to which leakage occurs, basedon rows 22 and 23 of the heat transfer pipes on which the discriminating marks are drawn.