A developer storage unit includes first and second rotary members. The first rotary member includes: first contacting portion capable of contacting with the second rotary member, first non-contacting portion not in contact with the second rotary member, and first stopper portion offset from the first contacting portion in an axial direction of the first rotary member. The second rotary member includes: second contacting portion capable of contacting with the first rotary member, second non-contacting portion not in contact with the first rotary member, second stopper portion offset from the second contacting portion in an axial direction of the second rotary member, and detected portion detected by detector. The second stopper portion comes into contact with the first stopper portion when the detected portion is moved from used product detecting position to new product detecting position while the first non-contacting portion is positioned to face the second rotary member.