To ensure the stable amount of feed water even if a cross flow rate is changed, facilitate control of residual chlorine concentration in the feed water, and reduce the amount of fouling material accumulating on the membrane surface of a membrane module in a cross flow membrane filtration method.

In a membrane filtration method for controlling a cross flow rate according to the turbidity of water to be treated, a flow rate of membrane filtrate is controlled at a constant level, and a prefilter which can be backwashed with backwashing water mixed with bubbles due to pressure air is installed in water to be treated inlet passage of the membrane module. A membrane filtration apparatus includes a means for controlling the cross flow rate according to the turbidity of the water to be treated, and a means for controlling the amount of the membrane filtrate at a constant level, and further provided with a pressure air injection opening for mixing the bubbles into the backwashing water, and the prefilter which can be backwashed with the backwashing water mixed with the bubbles in the water to be treated inlet passage of the membrane module.