Outer shape dimensions of a staple cartridge for binding 100 sheets and a staple cartridge for binding 50 sheets are made the same. Staple detecting holes are provided at bottom faces of both of them, and a cartridge identifying hole for identifying a type of a cartridge is formed at one of them. A cartridge identifying sensor and a staple detecting sensor are provided at the inside of an electric stapler. Charge of the staple cartridge, determination of a cartridge type and presence or absence of a staple at the inside of the staple cartridge are detected by signals of the sensors.