The present invention provides a variable speed accelerator including: a constant-speed motor (51) having a constant-speed rotor (52) which is configured to rotate a constant-speed input shaft (Ac) of a transmission device (10) in a first direction; and a variable-speed motor (71) which has a variable-speed rotor (72) connected to a variable-speed input shaft (Av) of the transmission device (10), having a cylindrical shape centered on an axis with a shaft insertion hole (74) passing therethrough in the axial direction through which the constant-speed input shaft (Ac) inserted, and configured to rotate an output shaft (Ao) at a maximum rotation rate by rotating the variable-speed rotor (72) at a maximum rotation rate in a second direction opposite to the first direction, wherein the variable speed accelerator further includes an AC power source line (110) which connects the variable-speed motor (71) with an AC power source to allow the variable-speed motor (71) to rotate in the second direction; a rotation rate controller (100) which is provided on the AC power source line (110) to control a rotation rate of the variable-speed motor (71); a first switch (SW1) provided on the AC power source line (110); and a braking circuit (2) connected to portions locating between the variable-speed motor (71) and the rotation rate controller (100) with the first switch (SW1) on the AC power source line (110).