In a biological optical measuring instrument, when displaying a biological variation in a predetermined region of a subject through irradiating light to the subject and processing light signals obtained by detecting the transmitting light, the light signals of a plurality of sections having a predetermined time interval are extracted, section data are prepared from the respective extracted light signals, the plurality of the prepared section data are added and averaged, prior to preparing averaged section data for the predetermined time interval a light signal section containing noise components is specified from differential values of the extracted light signals and the section data of the light signal containing the noise components is excluded from the averaging operation, thereby, a biological optical measuring instrument is provided which permits to remove the section data containing noise components are automatically removed through computation without relying upon visual observation, to obtain highly reliable measurement result quickly and to display the same.