PURPOSE: To discharge gas from a diffuser at steady velocity in the direction parallel with axis of a boss by having no line from one end of a blade to the other end run through the center point of an axis of the boss.

CONSTITUTION: A fan 4 has trapezoidal moving blades 13 generate blow toward the area near the rotary shaft 12, that losses a little pressure in the outlet of the fan 4 and leads air to a diffuser 3 in the rear of the fan. The location of each moving blade 13 fixed on the fan 4 is settled at an angle off from each other that projection of the blades on a boss 33 at right angle with the rotary shaft has the similar view as that of blades of a disc centrifugal fan, which is decided with respect from air quantity, turning number, and other conditions.