A voltage supply device 1 is provided with a primary cell 11 and a constant voltage supply unit 6 for controlling the output voltage V11 of the primary cell 11 and supplying the output voltage V11 to an absolute position detection device 4. A voltage reduction determination value A is set for the constant voltage supply unit 6 that is a threshold that is higher than a position data loss level C that is the voltage level at which the absolute position detection device 4 becomes incapable of retaining position data and lower than an output voltage V12 that is a constant voltage at a steady-state voltage level output by the constant voltage supply unit 6 during a steady state. The constant voltage supply unit 6 converts the output voltage V11 of the primary cell 11 to the output voltage V12 that is the voltage at the steady-state voltage level and supplies the same to the absolute position detection device 4 until the output voltage V11 of the primary cell 11 falls below the threshold. When the output voltage V11 of the primary cell 11 falls below the threshold, the constant voltage supply unit 6 converts the output voltage V11 of the primary cell 11 to an output voltage V13 that is a voltage at the same level as the threshold and supplies the same to the absolute position detection device 4.
