The N,N-di-substituted (meth)acrylamide as the component (b) used in the present invention includes an acyclic (meth)acrylamide of formula (2) wherein R4 and R5 are an alkyl group and a cyclic (meth)acrylamide of formula (2) wherein R4 and R5 combine with each other to form a heterocyclic ring with the N atom, and in the present invention, one or both of them are used according to the purpose of u