PURPOSE: To obtain an oligomer in a high conversion rate by bringing 10-24C straight-chain orefin, at a high temperature, with a catalyst containing halogenated phosphorus-containing acid on a montmorillonite clay as a substrate.

CONSTITUTION: Before montmorillonite clay is a catalyst for oligomerization, the clay is treated with harogenized phosphorus containing-acid (particularly, fluorophosphoric acid or difluorophosphoric acid). The oligomer is produced by bringing 10-24C straight-chain olefin into contact with the catalyst at a high temperature. In this method, the forming ratio of dimer to trimer is low and that of oligomer to trimer is high. As for catalyst, it is desirable that the residual acidity of montmorillonite clay before treating is smaller than 1 mgKOH/g and the pH is 7 or higher. Moreover, particularly, heterogeneous catalyst containing alkylsulfonate silica gel is desirable. The oligomer obtained is useful as a base oil of synthetic lubricant.