An attachment 10 that is proximal to an optical probe 9 includes probe information holding means 39 that holds probe information such as a scanning technique and an optical path length or the like. When the optical probe 9 is connected (attached) to an observing device 6, probe information detecting means included in the observing device 6 automatically detects the probe information held in the optical probe. Based on the detected probe information, a scanning technique and an optical path length of reference light are set to values suitable to an actually connected optical probe 9.