To provide a processing aid having good dispersibility in a styrene elastomer and improving moldability of the styrene elastomer, and a styrene elastomer composition containing the processing aid.

The processing aid for the styrene elastomer contains an alkyl (meth)acrylate polymer composed mainly of an alkyl (meth)acrylate unit having an alkyl group carbon number of 2 and having a mass-average molecular weight of 150,000-20,000,000. The styrene elastomer composition contains the processing aid and the styrene elastomer.