A method of measuring a welding-groove position for an over-lapped fillet welding of thin members, comprising a projection step of projecting a slit light so that the slit light is projected on both the thin members crossing a welding-groove face, a two-dimensional image acquisition step of capturing the slit-light image projected on each of the thin members as two-dimensional images; a three-dimensional conversion step of converting the two-dimensional images into three-dimensional images using a predetermined three-dimensional conversion parameter, and a calculation step of calculating the welding-groove position based on the three-dimensional data of the projected slit-light images, and relative positions between the projected slit-light images in the three-dimensional data defined by shapes and arrangement of the thin members.