To obtain a surface active compound having a surface activating ability and a polymerization-initiating ability.

This surface active compound is represented by the formula: [R1a-(B1)r-(X1-B2)t]p-(S)k-[(B3-X2)u-(B4)s-R2a]q (R1a is a unit having a hydrophobic group; R2a is a unit having a hydrophilic group; B1 and B4 are each a unit for binding R1 or R2 with active groups X1 or X2, B2 and B3 are each a unit for binding a spacer to X; (k), (r), (s), (t) and (u) are each 0 or 1; with the proviso that both of (t) and (u) are not simultaneously 0; and (p) and (q) are each an integer of 1-3). The active group is ordinarily the group generating a radical by heat or light.