Provided is a porous polymer flat-film sheet that is for MBR, has a porous polymer film surface that maintains hydrophilic properties over the long term, and maintains high water permeability and anti-fouling properties. The porous polymer flat-film sheet for MBR comprises a hydrophobic porous polymer film forming a network structure, and a sheet-shaped base material supporting the porous polymer film, and is characterized by the surface of at least the porous polymer film being covered by and adhered to insolubilized hydroxypropyl cellulose, the proportion of the hydroxypropyl cellulose to the porous polymer film being 0.4-1.0 wt%, and the ratio (B/A) of the strength of the spectrum peak (B) appearing at 4.12 ppm with respect to the strength of the spectrum peak (A) appearing at 3.75 ppm when using a nuclear magnetic resonance device (proton NMR); to measure the NMR spectrum of the hydroxypropyl cellulose covering and adhered to the film being 12-30%.