PURPOSE: To suppress exfoliation of a flow, by rotating a leading blade and qualizing a flow angle to an angle formed by a circumference and camber line to prevent an increase of an attack angle.

CONSTITUTION: Fluid 8 flows in along a camber line 5 of a guide vane 20, because an angle βb and angle β are almost equal, fluid 9 in the side of a pressure surface 6 and fluid 10 in the side of a negative pressure surface 7 flow respectively along the surfaces 6, 7. When a flow amount of the fluid decreases, the flow angle β is decreased smaller than at a design flow, and a piston 17 is pulled to rotate a seat 14 clockwise and move a leading blade 12 so as to equalize the angle β almost to the angle βb. Accordingly, an attack angle (βb-β) is never increased to prevent exfoliation of a flow.