PURPOSE: To check effectively the bowing of a lower manifold in light weight and simple structure without reducing greatly a flow passage cross area in the lower manifold.

CONSTITUTION: A lower manifold 10 is designed to have double structure which comprises an outer tube 10a and an inner tube 10b. The inner tube 10b is made of bellows with a thickness smaller than that of the outer tube 10a. This inner tube 10b is inserted into the outer tube 10a with a clearance 28 which is specified to remain between the inner and outer tubes where the clearance 28 is communicated with the inside of the inner tube.