PURPOSE: To efficiently reform a raw material stream containing light gas and a light naphtha C2 + olefin to a C5 + aliphatic hydrocarbon-containing heavy stream by bringing the same into contact with a zeolite metal silicate catalyst to regenerate the catalyst to circulate the same.

CONSTITUTION: When a fine divided silicate catalyst (A) and a raw material stream (B) containing light gas and a light naphtha C2 + olefin are brought into contact with each other, the main part of the component B is purged into the recirculated stream wherein the component A is fluidized by a non-liquid phase hydrocarbon and the purged catalyst is sent to a stripping region from a purge region. The purged catalyst is stripped and oxidatively regenerated to be brought to a state free from bonded coke. The regenerated catalyst is returned to an oligomerizing region to continuously reform the raw material to a C5 + aliphatic hydrocarbon.