A tape cartridge that is to be mounted on a tape printing device including a detecting portion for reading various types of information of the tape cartridge from a detected portion of the tape cartridge by applying detection light to the detected portion and capable of being attached to and detached from the tape printing device along a mounting direction, includes the detected portion disposed on a first wall which is formed along the mounting direction, and an engaging portion disposed on a second wall which is formed at a deep side of the mounting direction. The engaging portion is engaged with an engaged portion provided in a holder in which the detecting portion is disposed and performs positioning between the detecting portion and the detected portion via the holder.