A sample centering system comprising a photometric system for colorimetrically measuring samples applied onto a carrier shifted in a definite direction, a detection system arranged at a definite interval apart from said photometric system, a sample detection signal generating means for generating a signal on the basis of the output from said detection system, and a shifting timer which is operated on the basis of said sample detection signal or at a definite time after shifting of said carrier is started, said sample centering system being so adapted as to stop said carrier when said shifting timer is turned off after a definite time, thereby making it possible to stop said carrier always at a time when a sample is located on the optical axis of said photometric system regardless of sample conditions.