Integrally asymmetric flat membrane for microfiltration comprising at least 40 wt. % of a hydrophobic first sulfone polymer and a hydrophilic second polymer with a pore size distribution over the membrane wall, having a separating layer in the wall's interior, and also having, in the direction from this separating layer, pore sizes increasing towards the surfaces, the second surface having pores with a mean diameter of at least 1 μm. The membrane comprises the hydrophilic second polymer in a concentration of 0.1-10 wt. %. The separating layer is located in an region facing the first surface, and the pore size passes through a maximum in the direction from the asymmetrical region towards the second surface.Method for producing this membrane from a casting solution comprising the hydrophobic first sulfone polymer and the hydrophilic second polymer in a solvent system, the method comprising the steps of pouring the casting solution, conditioned to a molding temperature, onto a carrier to form a film, which carrier has a temperature that is higher in comparison to the molding temperature, conveying the film through a climate-controlled zone, initiating the coagulation in a coagulation bath for the formation of a membrane structure, withdrawing the membrane structure from the carrier with a speed that is increased in comparison to the carrier speed, stabilizing, extracting, and subsequently drying the membrane.