A butt position detecting method for a weld joint that detects a position of a butt portion, which is formed by butting a joint surface of a first member and a joint surface of a second member against each other, by irradiating the butt portion with irradiation light of a two-dimensional displacement sensor, characterized in that: when an arrangement direction of the first member and the second member is defined as a first direction, and a direction which is perpendicular to the first direction and in which the two-dimensional displacement sensor and the first member are arranged is defined as a second direction, the first member has a corner portion that is formed at an end of the joint surface of the first member in the second direction, and a chamfer portion that forms the corner portion in a boundary region with the joint surface of the first member; the second member has a corner portion that is formed at an end of the joint surface of the second member in the second direction, and a chamfer portion that forms the corner portion in a boundary region with the joint surface of the second member; the corner portion of the second member is arranged at a position that is located outside the corner portion of the first member with respect to the second direction; and the two-dimensional displacement sensor is arranged at a position offset from the butt portion toward the first member side in the first direction such that the chamfer portion of the second member, the corner portion of the second member, and the joint surface of the second member are irradiated with the irradiation light, and such that an angle θs at which the second direction and an optical axis direction of the irradiation light intersect with each other and an angle θa at which the second direction and a formation direction of the chamfer portion of the first member intersect with each other satisfy a condition expression: 0° < θs < θa, wherein the position of the butt portion is detected by specifying a position of the corner portion of the second member in a two-dimensional cross-sectional profile that is measured by the two-dimensional displacement sensor. when an arrangement direction of the first member and the second member is defined as a first direction, and a direction which is perpendicular to the first direction and in which the two-dimensional displacement sensor and the first member are arranged is defined as a second direction, the first member has a corner portion that is formed at an end of the joint surface of the first member in the second direction, and a chamfer portion that forms the corner portion in a boundary region with the joint surface of the first member; the second member has a corner portion that is formed at an end of the joint surface of the second member in the second direction, and a chamfer portion that forms the corner portion in a boundary region with the joint surface of the second member; the corner portion of the second member is arranged at a position that is located outside the corner portion of the first member with respect to the second direction; and the two-dimensional displacement sensor is arranged at a position offset from the butt portion toward the first member side in the first direction such that the chamfer portion of the second member, the corner portion of the second member, and the joint surface of the second member are irradiated with the irradiation light, and such that an angle θs at which the second direction and an optical axis direction of the irradiation light intersect with each other and an angle θa at which the second direction and a formation direction of the chamfer portion of the first member intersect with each other satisfy a condition expression: 0° < θs < θa, wherein the position of the butt portion is detected by specifying a position of the corner portion of the second member in a two-dimensional cross-sectional profile that is measured by the two-dimensional displacement sensor.