To provide an additive and an oil composition having good solubility to a base oil and good fluidity at a temperature.

This fluidity improving agent comprises a (co)polymer (A) having an alkoxyalkylene glycol money(meth)acrylate (a1) as an essential unit: represented by general R1-O-(XO)p-R2 [wherein R1 is a (meth)acryloyl group; R2 is an alkyl group; X is a 2-20C alkaline group p is a number of 1-10; X may be either the same or different when p is not less 2 and the (XO)p part may be either a random addition or a block addition].