Separation membrane and method for producing same

A separation membrane layer having a first surface for contacting liquid to be treated and a second surface disposed on the opposite side of the separation membrane layer through which treated liquid exits, wherein

• the separation membrane layer comprises solid spherical structures formed of a polyvinylidene fluoride-based resin,
• an average diameter D1 of the solid spherical structures S1 in a region within 10 μm from the first surface and an average diameter D2 of the solid spherical structures S2 in a region of 10 μm to 20 μm from the second surface satisfy a relational expression of D1>D2,
• the average diameter D1 and an average diameter D3 of the solid spherical structures S3 in a region of 10 μm to 20 μm from the first surface satisfy a relational expression of
• 10<D1/D3<4.00, and
• the separation membrane layer has an entire thickness of from 40 μm to 500 μm.