A power supply unit inclding a battery 4 that stores electric power used for operations of various devices; a dynamo 19, a rotation support mechanism 14, and a rectifying section 32 as charging means that converts natural energy into electric energy and supplies electric power of the electric energy to the battery 4 to be charged; a charging control section 36 for changing over between supply and stop of the electric power from the charging means to the storage means; and a charging control driving unit 45 and a low-voltage charging function of an arithmetic processing part 51 as charging changeover control means that controls the charging control section 36 such that supply and stop of the electric power are repeated when the charging voltage for charging the battery 4 with electric power is not less than a predetermined value, and continues supply of the electric power when the charging voltage is less than the predetermined value.