PURPOSE: To reduce cost by dividing blades of axially mutually opposite blade wheels into sectors distributed circumferentially, and arranging blades slanting in the direction opposite to the circumferential direction, succeeding to the sector having blades slanting in the circumferential direction.

CONSTITUTION: The blades 5 of a stator blade wheel in the drawing are divided into pairs of sectors A, B. The blades 5a in one sector A are slanting in one circumferential direction, while the blades 5b in the other sector B are slanting in the other circumferential direction. The blades of a rotor blade wheel are arranged in approximately the same way as those of the stator blade wheel 4. Thus, during the rotation of the rotor blade wheel, the position of trailing blades and the position of leading blades alternate. This equals to rotation in both directions. Therefore it is possible to produce an equal operation in both directions of rotation with only one pair of blade wheels.