A liquid density in a liquid supply tube, through which the liquid is supplied, is stably measured from the outside of the liquid supply tube. A liquid densitometer is provided with a liquid supply tube (14, 16) through which a liquid is supplied, a light transmission unit (15) set at a halfway point of the liquid supply tube (14, 16), a light projection unit (9, 22) that irradiates measurement light to the light transmission unit (15), a light reception unit (10, 23) that receives the measurement light to transmit through the light transmission unit (15), a support member (31) that supports movably the light projection unit (9, 22) and the light reception unit (10, 23) so as to move a measurement position (32) from which the light is irradiated to the light transmission unit (15) and at which the light reception unit (10, 23); receives the light transmitting through the light transmission unit (15), a measurement position moving mechanism (2) that makes the support member (31) move in order for the measurement position (32) to move in a predetermined area of the light transmission unit (15), and a data processing unit that acquires light intensity data which the light reception unit (10, 23) receives at a plurality of measurement positions (32) to calculate a density of the liquid, which flows through the liquid supply tube (14, 16), in accordance with the plurality of the light receiving intensities.