A variable speed accelerator includes: an electric driving device; and a transmission device changing the speed of a rotational driving force generated by the electric driving device and transmits the changed rotation driving force to a driving target. The transmission device includes: a sun gear; a sun gear shaft fixed to the sun gear; a planetary gear that meshes with the sun gear; an internal gear that meshes with the planetary gear; a planetary gear carrier including a planetary gear carrier shaft; and an internal gear carrier including an internal gear carrier shaft. The electric driving device includes: a constant-speed motor including a constant-speed rotor; and a variable-speed motor including a variable-speed rotor. The variable speed accelerator further includes: a rotation rate controller that issues a speed instruction; a torque measuring device; and a control device holding an instruction value of the speed instruction.