A voltage conversion circuit comprising: a plurality of primary capacitors (21, 22, 23) which are adapted to be charged by a power supply including a vibration power generation device for converting external vibration energy into electric power; a secondary capacitor which is connected in parallel to the respective plurality of primary capacitors (21, 22, 23) and is configured to be charged at a voltage supplied to a load circuit; a plurality of switching circuits (71a, 71b, 72a, 72b, 73a, 73b) which are provided in association with the respective plurality of primary capacitors (21, 22, 23) and are configured to change over a connection state between the primary capacitors (21, 22, 23) and the secondary capacitor; a connection control circuit (5, 41, 42, 43) configured to successively connect the respective primary capacitors (21, 22, 23) to the secondary capacitor through the corresponding switching circuits (71a, 71b, 72a, 72b, 73a, 73b), the respective primary capacitors (21, 22, 23) across which the charging voltages reach a predetermined connection voltage higher than a charging voltage across the secondary capacitor such that a voltage difference between the respective charging voltage across the respective primary capacitor (21, 22, 23) and the charging voltage across the secondary capacitor is a voltage difference which causes a charge-transfer efficiency from the respective primary capacitor (21, 22, 23) to the secondary capacitor to be equal to or higher than a predetermined efficiency; and an adjustment circuit (7, 21a, 21b, 22a, 22b, 23a, 23b) configured to adjust a connection state between the power supply and the plurality of primary capacitors (21, 22, 23), such that an adjustable number of primary capacitors (21, 22, 23) comprised in the plurality of primary capacitors (21, 22, 23) are in a state of being connected in series to the power supply, in order to maintain an efficiency of electric-power supply from the power supply to the plurality of primary capacitors (21, 22, 23) at a predetermined efficiency, wherein the efficiency of the electric-power supply varies depending on a relationship between an output voltage from the power supply and a total charging voltage across the primary capacitors (21, 22, 23) connected in series to the power supply. a plurality of primary capacitors (21, 22, 23) which are adapted to be charged by a power supply including a vibration power generation device for converting external vibration energy into electric power; a secondary capacitor which is connected in parallel to the respective plurality of primary capacitors (21, 22, 23) and is configured to be charged at a voltage supplied to a load circuit; a plurality of switching circuits (71a, 71b, 72a, 72b, 73a, 73b) which are provided in association with the respective plurality of primary capacitors (21, 22, 23) and are configured to change over a connection state between the primary capacitors (21, 22, 23) and the secondary capacitor; a connection control circuit (5, 41, 42, 43) configured to successively connect the respective primary capacitors (21, 22, 23) to the secondary capacitor through the corresponding switching circuits (71a, 71b, 72a, 72b, 73a, 73b), the respective primary capacitors (21, 22, 23) across which the charging voltages reach a predetermined connection voltage higher than a charging voltage across the secondary capacitor such that a voltage difference between the respective charging voltage across the respective primary capacitor (21, 22, 23) and the charging voltage across the secondary capacitor is a voltage difference which causes a charge-transfer efficiency from the respective primary capacitor (21, 22, 23) to the secondary capacitor to be equal to or higher than a predetermined efficiency; and an adjustment circuit (7, 21a, 21b, 22a, 22b, 23a, 23b) configured to adjust a connection state between the power supply and the plurality of primary capacitors (21, 22, 23), such that an adjustable number of primary capacitors (21, 22, 23) comprised in the plurality of primary capacitors (21, 22, 23) are in a state of being connected in series to the power supply, in order to maintain an efficiency of electric-power supply from the power supply to the plurality of primary capacitors (21, 22, 23) at a predetermined efficiency, wherein the efficiency of the electric-power supply varies depending on a relationship between an output voltage from the power supply and a total charging voltage across the primary capacitors (21, 22, 23) connected in series to the power supply.