To provide a pitch angle control device 13 of a windmill blade 11 almost unaffected even by a lightning strike though it is miniaturized.

This control device is so designed that the linear drive force of a piston rod 16 is converted into a rotating drive force while detecting the linear motion of the piston rod 16 by a detection roller 77 in rolling contact with the piston rod 16 of a fluid cylinder 14. Since the axial length of the detection part can be made shorter than that of a long ball screw shaft mentioned in a prior art, the entire control device can be miniaturized. In addition, since a portion for controlling a servo control valve 34 is not an electronic circuit but a mechanical configuration formed of a conversion/movement means, a control motor 59, a detection roller 77, and a mechanical transmission means, the control device is almost unaffected by the lightning strike.