Abstract: In an automatic analyzer such as blood analyzer, it is important to observe whether the sample, such as a blood sample, is sucked correctly or not. In an apparatus for observing suction of a sample, comprising a sample suction tube, a luminous element for emitting light toward the sample suction tube being driven by a driving circuit, a photo sensor for receiving the light from the sample suction tube, and a judging circuit for judging the sample suction on the basis of the signal from the photo sensor, a controller is provided in order to drive the luminous element so that the output (Vp) of the photo sensor may be nearly the reference value (Vr) before a sample suction, and to drive the luminous element so as to hold the driving state during the sample suction. Thus, the sensitivity adjustment of the apparatus for observing suction of a sample may be automated, and the observing capability of the apparatus is enhanced.