PURPOSE: To firmly fix a separator plate and a gasket without a risk of their falling off of the separator groove area even if the thickness of a pipe plate of a heat exchanger is reduced to the minimum by installing a guide support in a gasket branch groove disposed in the water chamber side pipe plate of the heat exchanger.

CONSTITUTION: A groove is formed to a depth of l3 in an area where a gasket 6 is inserted to a pipe plate 3. The depth of the groove is made barely enough to fit a support guide 7 to the groove, and may as well be 2mm which is smaller than the thickness of the gasket 6. A guide support 7 is attached to the pipe plate surface with a fastening screw 8 on both sides of a separator wall 2 to prevent the deformation of the gasket 6 and separator plate 2. The guide support 7 is given a sufficient thickness l4 in consideration of the deformation etc. of the pipe plate surface. By such an arrangement, the thickness of the pipe plate 3 is T1+l3 which is minimum yet sufficient. Since there is a guide support 7 on either side of the separator plate 2, its deformation can be prevented even if a load P is applied to the separator plate 2 due to the differential pressure. The wall thickness of the pipe plate can be reduced to the minimum, and the gasket 6 and separator plate 2 can firmly be fixed to the groove without falling off.