The present invention is a method for producing an internal olefin, wherein an .alpha.-olefin containing 50 ppm water or less according to a Karl Fischer test method is contacted with a zeolite catalyst and/or a montmorillonite catalyst for the isomerization thereof and provides an industrially advantageous method for the selective internal isomerization of the raw material .alpha.-olefin using a inexpensive catalyst while preventing catalyst deterioration and side reactions such as skeletal isomerization, oligomerization and cracking.