A dose measuring device for a drug delivery device is provided. The dose measuring device includes a main body and a dial sleeve coupled to the main body to spirally move, the dose measuring device including a housing provided to be removable from the main body, a rotation ring received in the housing to rotate, an encoder installed in the housing to detect the amount of rotation of the rotation ring, and an auxiliary sleeve provided to be removable from the dial sleeve and to spirally move together with the dial sleeve in the housing, in which the rotation ring is bound to the auxiliary sleeve to rotate together with the auxiliary sleeve and is fixed in the housing in a longitudinal direction of the main body.
