An injection-device and an injection-measuring method are described. The injection-device includes a pen-type sleeve for accommodating a cartridge holder, a mechanical injection system provided in the pen-type sleeve, which has a dosing sleeve by which a dosage quantity to be injected is predefinable and which is configured to execute a rotatory motion during the injection process； a selector device, connected to the dosing sleeve, for adjusting the dosage quantity to be injected； an injection button, connected to the selector device, for applying an injection force； and an optical sensor device for detecting the rotatory motion of the dosage sleeve during injection of the dosage quantity, which has a signal-processing device for ascertaining and storing the injected dosage quantity based on the detected rotatory motion of the dosage sleeve. The injection button has a switch device by which the optical sensor device is activatable when applying the injection force.
