PURPOSE: To easily inspect a bypass leakage at a connecting section of a partition in a tank an heat exchanger.

CONSTITUTION: A detecting passage 50 for detecting bypass leakage is formed between a partition 46 and an end plate 4, a leakage detecting hole 52 opened out of a heat exchanger is provided to communicate with the passage 50 at a predetermined position (cap 38 of a tank 8) surrounding the passage 50, and a bypass leakage is inspected according to whether a pressure leakage occurs or not through the passage 50 when a pneumatic pressure of a predetermined value is applied to the interior of the heat exchanger. The hole 52 is blocked by a torch brazing, etc., after the inspection is finished.