A catalyst which is able to express a high oligomerizing activity of ethylene and a method for the production of &agr;-olefin where ethylene is oligomerized using said catalyst are provided. The present invention relates to a catalyst for the production of &agr;-olefin obtained by contacting (a) clay, clay mineral or ion-exchange layer compound with (b-1) a transition metal complex of Groups 4 to 6 of the Periodic Table and also relates to a method for the production of &agr;-olefin by oligomerization of ethylene using said catalyst. The present invention further relates to a catalyst for the production of &agr;-olefin obtained by contacting (a) clay, clay mineral or ion-exchange layer compound with (b-2) a transition metal complex of Groups 8 to 10 of the Periodic Table and also relates to a method of the production of &agr;-olefin by oligomerization of ethylene using said catalyst.