A magnetic separation unit can prevent any magnetic flocs deposited on an inner wall of a water passing pipe from closing a water channel for stabilizing a magnetic separation performance and reduce cost by a small-diameter portion of the water passing pipe. The magnetic separation unit has an air-core solenoid type magnet provided around the water passing pipe guiding, from a lower side, treated water including magnetic flocs in which contaminants and magnetic particles are flocculated, a rotating disc type magnetic filter case provided above the air-core solenoid type magnet for attracting the magnetic flocs in the treated water supplied from the water passing pipe and flushing means for flushing the matrixes to which the magnetic flocs are attracted. The water passing pipe has a small-diameter pipe positioned in the air-core solenoid type magnet, and an enlarged pipe enlarged from the small-diameter pipe toward a water passing part of the rotating disc type magnetic filter case.