To provide a windmill improved in starting characteristic by smoothing of torque variation.

It comprises a rotary shaft 2 rotatably supported, a first partition plate 3A, a second partition plate 3B, a third partition plate 3C and a fourth partition plate 3D fixed to the rotary shaft 2, a first rotary vane unit 4U1 having a pair of vanes 5U1, 5U1 of a semi-cylindrical shape with their upper sides fixed to the first partition plate 3A and their lower sides fixed to the second partition plate 3B, a second rotary vane unit 4U2 having a pair of vanes 5U2, 5U2 of a semi-cylindrical shape with their upper sides fixed to the second partition plate 3B and their lower sides fixed to the third partition plate 3C, and a third rotary vane unit 4U3 having a pair of vanes 5U3, 5U3 of a semi-cylindrical shape with their upper sides fixed to the third partition plate 3C and their lower sides fixed to the fourth partition plate 3D. Mounting angles of the first rotary vane unit 4U1, the second rotary vane unit 4U2 and the third rotary vane unit 4U3 to the rotary shaft 2 are shifted by 120 degrees from each other.