The semiconductor device includes the charging system including: electric power generating unit for supplying electric power; electric power storing unit for storing electric power generated by the electric power generating unit; switch unit provided in a charging path for charging the electric power storing unit with the electric power generated by the electric power generating unit; a comparator driven by the electric power generated by the electric power generating unit for comparing a reference voltage and a stored voltage of the electric power storing unit; and a level converter provided between the comparator and the switch unit for, based on a result of a comparison made by the comparator, converting a level of a generated voltage to a level of the stored voltage and outputting a resultant signal to the switch unit.