Provided is an apparatus and method for measuring a dose in an injector, which is capable of measuring a dose without modifying the mechanical structure of the injector. A number sleeve (150) is provided to pass through an injector body (100) and is connected to be spirally movable, and a pattern (151,152,153) for dose measurement is formed on an outer periphery of the number sleeve. The injector body includes a sensor (210,410,610) for sensing the pattern formed on the number sleeve when the number sleeve performs a spiral movement, and a controller (215,415,615) for measuring a dose according to a spiral movement distance of the number sleeve through the sensor.
