Separation membrane and method for producing separation membrane

A separation membrane comprising, as a main component A, a thermoplastic polymer selected from the group consisting of a cellulose ester and a polyamide,

• wherein, when regions obtained by dividing a cross-sectional surface perpendicular to a longitudinal direction of the separation membrane into 5 at an equal interval in order from one surface of the separation membrane in a thickness direction of the separation membrane are defined as regions 1 to 5,
• an average pore diameter obtained by weighting with a pore area ratio of pores extracted in each region to a total area of all the pores is defined as an area average pore diameter Ds in each region,
• an average pore diameter obtained by weighting with a number ratio of pores extracted in each region to the number of all pores is defined as a number average pore diameter Dn in each region,
• a pore having a pore diameter larger than the area average pore diameter Ds is defined as a coarse pore,
• a pore having a pore diameter smaller than the number average pore diameter Dn is defined as a fine pore,
• an average of closest distance between coarse pores is defined as La, and
• a number average W of the fine pores that are located at a distance smaller than La from a center of the respective coarse pores is calculated,
• all the regions 1 to 5 have a number average pore diameter changing rate a defined by the follow Formula of −0.25 or more and 0.25 or less, and
• at least one of the regions 1 to 5 is a region P that satisfies the following conditions (a) and (b):
• (a) a value of the area average pore diameter Ds/the number average pore diameter Dn is 2.50 or more and 6.00 or less;
• and
• (b) the number average W of the fine Pores is 10 or more and 30 or less,
• αi=(Di−Di+1)/Di
• (Di:
• Number average pore diameter of region i),
• it is noted that the number average pore diameter changing rate in the region 5 is defined by the following formula,
• α5=(D5−D4)/D5