A voltage supply apparatus includes: a primary cell; and a constant voltage supply unit. The constant voltage supply unit has set therein a voltage drop determination value, which is a threshold that is higher than a positional-data loss level and is lower than a second output voltage, which is a constant voltage at a steady voltage level that the constant voltage supply unit outputs when in a steady state. The constant voltage supply unit converts a first output voltage of the primary cell to the second output voltage and supplies it to the absolute-position detection apparatus until the first output voltage falls below the threshold. When the first output voltage falls below the threshold, the constant voltage supply unit converts the first output voltage to a third output voltage which is a voltage at the same level as the threshold, and supplies it to the absolute-position detection apparatus.