This apparatus (20) for concentrating a microalga culture fluid has: a separation tank (21) for holding a microalga culture fluid which contains a microalga； a light irradiation unit (22) for irradiating the microalga culture fluid held in the separation tank (21) with a ray of light having a wavelength capable of attracting the microalga； and a take-out pipe (24) for taking out a concentrated culture fluid, namely, a microalga-rich part which is a part of the resulting microalga culture fluid and which has a microalga concentration enhanced by light irradiation. Thus, the present invention makes it possible to concentrate a microalgae culture fluid which contains a microalga at a low cost and with high efficiency, thereby enabling effective utilization of the microalga and culture fluid recovered after separation subsequent to the concentration.
