A biological observation apparatus comprising: a sound wave radiating unit configured to radiate an ultrasound wave into an object to be examined in living tissue at a scan position in a plane normal to a depth direction in the living tissue; a driving signal generator configured to output a timing signal to a light radiating unit, which signal indicates the timing at which an ultrasound wave signal is output at the scan position; the light radiating unit being configured to radiate light into the object immediately after the timing signal; a light interfering unit configured to make the light radiated from the light radiating unit interfere with light reflected from an area where the ultrasound wave reaches and radiates interference light; a light detector configured to detect the interference light radiated from the light interfering unit and output an interference signal corresponding to the interference light; and a calculation unit configured to: - calculate the amount of frequency modulation of the radiated light based on the interference signal outputted from the light detector and on the timing at which the timing signal is received by the calculation unit, and - store the amount of frequency modulation in a memory comprised in the calculation unit, characterized in that: the calculation unit is further configured to sequentially calculate a difference value between the amounts of frequency modulation of two areas which are adjacent to each other in the depth direction in the living tissue, wherein the calculation unit is configured to: - relate the difference value to the scan position with respect to the living tissue, - store the difference value and the scan position, and - generate a tomographic image of the object based on the difference value and the scan position, wherein the the sound wave radiating unit is positionable such that the ultrasound wave and the light are radiated in the same depth direction of the living tissue, and wherein the sound wave radiating unit is configured to radiate the ultrasound wave as a cyclic compressional wave whose frequency successively changes. a sound wave radiating unit configured to radiate an ultrasound wave into an object to be examined in living tissue at a scan position in a plane normal to a depth direction in the living tissue; a driving signal generator configured to output a timing signal to a light radiating unit, which signal indicates the timing at which an ultrasound wave signal is output at the scan position; the light radiating unit being configured to radiate light into the object immediately after the timing signal; a light interfering unit configured to make the light radiated from the light radiating unit interfere with light reflected from an area where the ultrasound wave reaches and radiates interference light; a light detector configured to detect the interference light radiated from the light interfering unit and output an interference signal corresponding to the interference light; and a calculation unit configured to: - calculate the amount of frequency modulation of the radiated light based on the interference signal outputted from the light detector and on the timing at which the timing signal is received by the calculation unit, and - store the amount of frequency modulation in a memory comprised in the calculation unit, - calculate the amount of frequency modulation of the radiated light based on the interference signal outputted from the light detector and on the timing at which the timing signal is received by the calculation unit, and - store the amount of frequency modulation in a memory comprised in the calculation unit, characterized in that: the calculation unit is further configured to sequentially calculate a difference value between the amounts of frequency modulation of two areas which are adjacent to each other in the depth direction in the living tissue, wherein the calculation unit is configured to: - relate the difference value to the scan position with respect to the living tissue, - store the difference value and the scan position, and - generate a tomographic image of the object based on the difference value and the scan position, wherein the the sound wave radiating unit is positionable such that the ultrasound wave and the light are radiated in the same depth direction of the living tissue, and wherein the sound wave radiating unit is configured to radiate the ultrasound wave as a cyclic compressional wave whose frequency successively changes. the calculation unit is further configured to sequentially calculate a difference value between the amounts of frequency modulation of two areas which are adjacent to each other in the depth direction in the living tissue, wherein the calculation unit is configured to: - relate the difference value to the scan position with respect to the living tissue, - store the difference value and the scan position, and - generate a tomographic image of the object based on the difference value and the scan position, - relate the difference value to the scan position with respect to the living tissue, - store the difference value and the scan position, and - generate a tomographic image of the object based on the difference value and the scan position, wherein the the sound wave radiating unit is positionable such that the ultrasound wave and the light are radiated in the same depth direction of the living tissue, and wherein the sound wave radiating unit is configured to radiate the ultrasound wave as a cyclic compressional wave whose frequency successively changes.