A method for determining whether a testing specimen is supplied to a testing system, which includes a reaction tank in which the testing specimen reacts; a light source for emitting testing light into the reaction tank from one side of the reaction tank; and a photoelectric sensor that is placed on the side of the reaction tank opposite to the light source for receiving the testing light traveled through the reaction tank and that converts the received testing light into an electrical signal. The amount of the testing specimen to be added into the reaction tank is set so that the level of the testing specimen is above the light path of the testing light while the reaction tank is cleaned. Whether the testing specimen is supplied is determined based on the presence of change of the electrical signal output from the photoelectric sensor.