An interchangeable lens barrel and a program rewriting system, in which a program included in the interchangeable lens barrel is easily changeable, are provided. An interchangeable lens barrel 2 is detachable from a camera body 1, and includes a storage section 36 for storing a first program concerning a function of the interchangeable lens barrel 2, a connection section 38 for connecting to an external input / output device 60, a reception section 32 for receiving a second program concerning the function of the interchangeable lens barrel from the input / output device 60 through the connection section 38, and a rewriting section 37 for replacing the stored first program with the received second program.