PURPOSE: To promote the orientation of a pump to high pressure ratio, by dividing the inside of a main flow path of the pump into plural parts by internal guide plates, which reduce a flow speed component in the rotary direction, and providing separate spaces.

CONSTITUTION: A plurality of internal guide plates 15, which reduce a flow speed component in the rotary direction of an impeller 9, are arranged. Gas, while it performs a helical motion in the peripheral direction in a housing 4 by the rotary action of the impeller 9, is fed to be transferred reaching a delivery port 13. The gas, while its flow is forcedly changed in a direction of reducing the flow speed component by the internal guide plates 15, increases the pressure at each time, and the gas, in a condition that its distance advancing in the peripheral direction decreases shorter during one turn in the housing 4, is guided by a guide member 14 being compressed in stages. Accordingly, the orientation of a pump to high pressure ratio can be attained by increasing the number of boost times.