A surfactant (a) having a critical micelle concentration (CMC) of 0.10 g/L or lower, represented by formula (1): R1—[—O-(A1O)n—H]m (1), wherein R1 is a C7-C20 hydrocarbon group having a valence number of m; m is an integer of 2 to 6; A1 in (A1O)n in each of [—O-(A1O)n—H]m is each independently a C2-C4 alkylene group; n in [—O-(A1O)n—H]m is each independently an integer of 1 to 100; and the sum of n in [—O-(A1O)n—H]m is 13 or greater.