To provide a charging system for a lithium ion battery, in which a proper overcharging determination value can be set according to a battery state value in charging, in order to more accurately determine battery overcharging.

The charging system 200 includes: a battery pack 20; and a charging device 1 which charges a secondary battery cell 21 with its voltage and current set to a predetermined value and has a charging control circuit for stopping charging when the charging voltage of the battery pack reaches an overcharge determination value (X-). The charging system 200 also has a battery voltage detection means 23 for detecting the battery voltage of the battery cell 21; a battery state detection means 28 for detecting a state value (Cc) of the battery cell 21 relating to variation in the overcharged state; and an overcharge determination value setting means 29 (microcomputer 14) for setting an overcharge determination value (X-) used to determine that the battery pack 20 is in an overcharged state. The overcharge determination value setting means 29 sets the overcharge determination value (X-) in response to the state value (Cc) of the battery cell 21 detected by the battery state detection means 28.