PURPOSE: To selectively produce an internal olefin having higher stability by isomerizlng an olefin using a solid base catalyst prepared by a specific method.

CONSTITUTION: A solid base 18 produced by pretreating alumina with an alkali metal hydrogen compound and an alkali metal carbonate or aluminate in the atmosphere of an inert gas and subjecting the pretreated alumina to heat treatment at 200-450°C, preferably at 250-400°C. An olefin is allowed to isomerize at -30 to 120-°C, preferably at -10 to 100°C using the solid base as a catalyst to obtain an inner olefin whose double bond is isomerized to the more stable position. This solid base catalyst has an extremely high isomerizing activity even without an auxiliary, e.g. ammonia or hydrazine and can extremely efficiently complete the isomerization even with a small amount of the catalyst. Further, the objective internal olefin is selectively produced accompanying little by-product, e.g. a polymer and the more removal of the catalyst directly leads to the internal olefin having high purity.