A separation membrane comprising a polyether polymer having a weight-average molecular weight of 10
4
to 10
7
and obtained by polymerizing an oxirane compound of the formula:
1
wherein R is a hydrogen atom;
an alkyl group, an alkyl group having an halogen atom, or —CH
2
O(CH
2
CH
2
O)
k
—R′ (R′ is a group selected from an alkyl group, an alkenyl group, a cycloalkyl group, an aryl group and an aralkyl group; k is from 0 to 12.); or
an ethylenically unsaturated group, a reactive silicon-containing group or a methylepoxy-containing group, is excellent in mechanical strength and flexibility.