A rotation detection device comprising: a rotation detection circuit; a step-up power supply circuit; a step-down power supply circuit; a first power supply path; and a second power supply path, wherein the rotation detection circuit is configured to detect a rotation number of a motor that generates a torque applied to a steering mechanism of a vehicle, based on an electric signal (Se), wherein the electric signal (Se) is generated according to a rotation angle (θm) of the motor that is acquired through an in-vehicle sensor, wherein the step-up power supply circuit is configured to perform a step-up operation so that a voltage of a DC power supply mounted in the vehicle is maintained at a normal level, when a vehicle power supply is turned on and the voltage of the DC power supply is lowered below the normal level, wherein the step-down power supply circuit is connected to the DC power supply and is configured to perform a step-down operation to a suitable voltage for operation of the rotation detection circuit, wherein the first power supply path connects the DC power supply and the step-down power supply circuit, wherein the second power supply path connects the step-up power supply circuit and the step-down power supply circuit, wherein power is supplied to the step-down power supply circuit via the first power supply path, when the vehicle power supply is turned on and the voltage of the DC power supply is maintained at the normal level, and wherein the power is supplied to the step-down power supply circuit via the second power supply path, when the vehicle power supply is turned on and the voltage of the DC power supply is lowered below the normal level, characterized in that the step-up power supply circuit is configured to stop operation when the vehicle power supply is turned off, and configured to operate so as to generate a constant voltage by switching the step-up operation and the step-down operation according to the voltage of the DC power supply when the vehicle power supply is turned on, wherein the rotation detection device includes another step-down power supply circuit (72a - 72n) configured to stop operation when the vehicle power supply is turned off, and configured to operate so as to lower a voltage generated by the step-up power supply circuit to the suitable voltage for another power supply target except for the rotation detection circuit when the vehicle power supply is turned on, and wherein the voltage generated by the step-up power supply circuit is supplied to the other step-down power supply circuit (72a - 72n) through a third power supply path other than the first power supply path and the second power supply path when the vehicle power supply is turned on. a rotation detection circuit; a step-up power supply circuit; a step-down power supply circuit; a first power supply path; and a second power supply path, wherein the rotation detection circuit is configured to detect a rotation number of a motor that generates a torque applied to a steering mechanism of a vehicle, based on an electric signal (Se), wherein the electric signal (Se) is generated according to a rotation angle (θm) of the motor that is acquired through an in-vehicle sensor, wherein the step-up power supply circuit is configured to perform a step-up operation so that a voltage of a DC power supply mounted in the vehicle is maintained at a normal level, when a vehicle power supply is turned on and the voltage of the DC power supply is lowered below the normal level, wherein the step-down power supply circuit is connected to the DC power supply and is configured to perform a step-down operation to a suitable voltage for operation of the rotation detection circuit, wherein the first power supply path connects the DC power supply and the step-down power supply circuit, wherein the second power supply path connects the step-up power supply circuit and the step-down power supply circuit, wherein power is supplied to the step-down power supply circuit via the first power supply path, when the vehicle power supply is turned on and the voltage of the DC power supply is maintained at the normal level, and wherein the power is supplied to the step-down power supply circuit via the second power supply path, when the vehicle power supply is turned on and the voltage of the DC power supply is lowered below the normal level, characterized in that the step-up power supply circuit is configured to stop operation when the vehicle power supply is turned off, and configured to operate so as to generate a constant voltage by switching the step-up operation and the step-down operation according to the voltage of the DC power supply when the vehicle power supply is turned on, wherein the rotation detection device includes another step-down power supply circuit (72a - 72n) configured to stop operation when the vehicle power supply is turned off, and configured to operate so as to lower a voltage generated by the step-up power supply circuit to the suitable voltage for another power supply target except for the rotation detection circuit when the vehicle power supply is turned on, and wherein the voltage generated by the step-up power supply circuit is supplied to the other step-down power supply circuit (72a - 72n) through a third power supply path other than the first power supply path and the second power supply path when the vehicle power supply is turned on.