To provide an external corner member with excellent workability, which can be constructed on an external corner portion of a building without making a noticeable gap formed in abutting portion and without using any sealing material, a constructing structure of external corner portion using the external corner member, and a construction method of external corner portion. The present invention is an external corner wall material, which is obtained by disposing two plate material at substantially a right angle, and which has abutting portion that is obtained by abutting an inclined end surface of each plate material to an inclined end surface of the right or left external wall board disposed adjacent to the plate material, and a cutout portion that is provided behind the abutting portion. The present invention includes also a base joiner having a positioning guide for attaching external wall, a constructing structure of external corner portion using the external corner wall material and the base joiner, and a construction method of external corner portion. The external corner wall material and external wall board are abutted to each other, and the positioning guide for attaching external wall is disposed in a cutout portion formed in an end portion of the external corner wall material.