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.