The penetrating members (21, 35, 37, 38, 39) are disposed inside the cooling passage (8) at positions so as to penetrate through the cooling passage (8), and have both of ends thereof fixed to the rear side wall (17) and the body side wall (18). The projecting members (33) (34A, 34B) are disposed inside the cooling passage (8) so as to project from the rear side wall (17) or the body side wall (18). The rectification ribs (36) are disposed at intermediate positions between the penetrating members (21, 35, 37, 38, 39) or the projecting members (33, 34A, 34B). The penetrating members (21, 35, 37, 38, 39) or the projecting members (33, 34A, 34B) disturbs the main stream (3a) of the cooling fluid (3) near the internal partition (19) in both directions of the leading edge wall (16) and the first internal partition (19) and make the main stream (3a ) non-uniformly or causes its channelling.