An optical element alignment method, comprising: preparing at least one optical element; preparing a baseplate including: a set position for mounting the at least one optical element, a first reference hole (A'), and a second reference hole (B') spaced apart in a first direction from the first reference hole by a first distance; and preparing an optical-fiber-fixing block configured to mount at least an optical fiber optically coupled with the optical element and a lens unit in a fixed manner; wherein the set position is determined by a first baseline that passes through the first reference hole and the second reference hole and by a second baseline that intersects with the first baseline at a second distance from the first reference hole, the set position being located at the intersection of the first baseline and the second baseline, the second distance being either in the first direction or in a second direction which is opposite to the first direction; wherein the optical-fiber-fixing block includes: a first post (C') configured to be inserted into the first reference hole, and a second post (D') configured to be inserted into the second reference hole; wherein a line passing through the first post and the second post is defined as a third baseline 350'; wherein the optical-fiber-fixing block is installed so that the second post is inserted into the second reference hole in a looser manner than the first post is inserted into the first reference hole, causing the second post (D') to be movable in the second reference hole (B') and the third baseline 350' to be movable in the clockwise direction or in the counterclockwise direction centering around the first reference hole (A'); and wherein the second distance is shorter than the first distance. preparing at least one optical element; preparing a baseplate including: a set position for mounting the at least one optical element, a first reference hole (A'), and a second reference hole (B') spaced apart in a first direction from the first reference hole by a first distance; and a set position for mounting the at least one optical element, a first reference hole (A'), and a second reference hole (B') spaced apart in a first direction from the first reference hole by a first distance; and preparing an optical-fiber-fixing block configured to mount at least an optical fiber optically coupled with the optical element and a lens unit in a fixed manner; wherein the set position is determined by a first baseline that passes through the first reference hole and the second reference hole and by a second baseline that intersects with the first baseline at a second distance from the first reference hole, the set position being located at the intersection of the first baseline and the second baseline, the second distance being either in the first direction or in a second direction which is opposite to the first direction; wherein the optical-fiber-fixing block includes: a first post (C') configured to be inserted into the first reference hole, and a second post (D') configured to be inserted into the second reference hole; wherein the set position is determined by a first baseline that passes through the first reference hole and the second reference hole and by a second baseline that intersects with the first baseline at a second distance from the first reference hole, the set position being located at the intersection of the first baseline and the second baseline, the second distance being either in the first direction or in a second direction which is opposite to the first direction; wherein the optical-fiber-fixing block includes: a first post (C') configured to be inserted into the first reference hole, and a second post (D') configured to be inserted into the second reference hole; a first post (C') configured to be inserted into the first reference hole, and a second post (D') configured to be inserted into the second reference hole; wherein a line passing through the first post and the second post is defined as a third baseline 350'; wherein the optical-fiber-fixing block is installed so that the second post is inserted into the second reference hole in a looser manner than the first post is inserted into the first reference hole, causing the second post (D') to be movable in the second reference hole (B') and the third baseline 350' to be movable in the clockwise direction or in the counterclockwise direction centering around the first reference hole (A'); and wherein the second distance is shorter than the first distance. wherein the optical-fiber-fixing block is installed so that the second post is inserted into the second reference hole in a looser manner than the first post is inserted into the first reference hole, causing the second post (D') to be movable in the second reference hole (B') and the third baseline 350' to be movable in the clockwise direction or in the counterclockwise direction centering around the first reference hole (A'); and wherein the second distance is shorter than the first distance.