In a probe positioning technology, an optical bioinstrumentation includes a region selecting unit that is used to delineate a region of interest in an anatomical image of a subject, a computing unit that determines a recommended probe position according to the region of interest, a probe position sensor that detects a current probe position, a computing unit that calculates the distance between the recommended probe position and the current probe position, and an alarm device that generates an alarm sound or the like when the distance falls within a predetermined range. Moreover, the optical bioinstrumentation for living body further includes a memory unit in which the probe position is saved together with measurement data.