To provide a suction device capable of following fluctuation of a water level while a suction port is fixed without requiring a space occupied by a float, and to provide an oil-water separation device and an oil-water separation tank having the suction device.

A suction device includes: a hollow support 3 with one end connected to a suction means; a hollow suction body 5 fitted into the support 3 slidably in a vertical direction; and a float 6 mounted in the vicinity of an end opposite to an end fitted into the support 3 of the suction body 5.