The present invention relates to a separation membrane including a thermoplastic polymer selected from a cellulose ester and a polyamide, in which, when regions obtained by dividing a cross-sectional surface perpendicular to a longitudinal direction of the separation membrane into 5 at an equal interval are defined as regions 1 to 5, all the regions 1 to 5 have a number average pore diameter changing rate αi of −0.25 to 0.25, and at least one of the regions 1 to 5 is a region P that satisfies conditions (a) and (b): (a) a value of area average pore diameter Ds/number average pore diameter Dn is 2.50 to 6.00; and (b) a number average W of fine pores that are located at a distance smaller than La from a center of respective coarse pores is 10 to 30.