To improve the performance of a diagonal flow compressor using a diffuser by blowing a flow along a main flow (blade) into a separation region to reduce the separation region without generating disturbance in the flow.

The diffuser 1 for a wind powered machine is mounted on a rotor of the diagonal flow compressor, and comprises a first blade 2 located on the upstream side with reference to the flowing direction of fluid, and a second blade 3 located on the downstream side. The downstream end of the first blade 2 is located on the downstream side of the upstream end of the second blade 3, and a narrow flow path S is formed between the first blade 2 and the second blade 3.