The purpose of the present invention is to provide a separation membrane module of which the performance does not deteriorate even when coming into contact with a biological component such as blood, which can be used for a prolonged period, which has a superior removal performance against moisture and the like, and which produces less eluates. The present invention is provided with a separation membrane comprising a hydrophobic polymer, a hydrophilic polymer, and polymer A, wherein: polymer A is a copolymer that comprises a hydrophilic unit and a hydrophobic unit and that has an alkyl group having 2-20 carbon atoms at a side-chain terminal of the hydrophilic unit； and when two liters of bovine blood containing 50 U/ml of heparin and having a hematocrit value of 30 vol% and a total protein concentration of 6-7 g/dl is circulated at a flow rate of 100 ml/min at 37°C so as to result in a filtration flow rate of 10 ml/(min·m2), the albumin sieving coefficient as of 60 minutes after the initiation of the circulation retains at least 86% of the albumin sieving coefficient as of 10 minutes after the initiation of the circulation.
