A living body observation apparatus irradiates a predetermined region inside a living body with ultrasound, irradiates the predetermined region, which is irradiated with the ultrasound, with a light of a predetermined frequency, and detects a scattered light with a frequency shifted by ultrasound vibration in a living body constituent substance included in the predetermined region. Velocity information of the living body constituent substance is calculated from a frequency shift amount in a detection signal of the scattered light, a density of the living body constituent substance is calculated from the velocity information, and discrimination of a state of the living body constituent substance is performed by using a value of the calculated density.
