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.