Lens unit, LED head, exposure device, and image formation apparatus

A method of manufacturing a lens unit comprising:

• preparing a lens plate member including a plurality of lenses arranged in a first direction;
• and a light block member including a plurality of opening portions arranged in the first direction and provided in one-to-one correspondence with the lenses, wherein a first positioning portion is formed at a first position in the first direction of each of the lens plate member and the light block member such that, in a state in which the lens plate member and the light block member are stacked together, the first positioning portion of the lens plate member and the first positioning portion of the light block member continuously extend in a second direction along an optical axis direction of the lenses, and wherein the lens plate member includes a plurality of second positioning portions arranged in the first direction and the light block member includes a plurality of second positioning portions arranged in the first direction, such that in the state where the lens plate member and the light block member are stacked together, the second positioning portions of the lens plate member and the second positioning portions of the light block member continuously extend in the second direction;
• and
• stacking the lens plate member and the light block member by applying adhesive between the lens plate member and the light block member and using an assembly jig including a first abutment portion contacting the first positioning portions and second abutment portions contacting the second positioning portions, such that the first positioning portions contact the first abutment portion of the assembly jig to position the lens plate member with respect to the light block member in the first direction and the second positioning portions contact the second abutment portions of the assembly jig to position the lens plate member with respect to the light block member in a third direction orthogonal to the first direction and the second direction.