An immersion type membrane filter apparatus 200 is intended for obtaining filtrate by filtrating a fluid to be processed stored within a storage bath 150 and primarily includes a filtration membrane module 300 and an air bubbles supply device 500 disposed therebelow. The filtration membrane module 300 has numerous tubular membranes 310 having on the inner surfaces thereof the function of filtrating a fluid to be processed packed within a cylindrical housing vessel 301 having a discharge port 303 for filtrate. The tubular membranes 310 are held in an opened state at both the ends of the housing vessel 301. The filtration membrane module 300 is soaked in a fluid to be processed in such a way that the tubular membranes 310 are opened in the vertical direction. When air bubbles are supplied to the filtration membrane module 300 from an air bubbles supply device 500, the fluid to be processed flows through the tubular membranes 310 from the down side upwards to naturally circulate within the storage bath 150. Part of the fluid to be processed is filtrated by passing through the tubular membranes 310 from the inside to the outside. As a result, the filtrate thus obtained flows out via the discharge port 303 to a filtrate discharge pathway 600.