A projection method, which is executed by a projection device, for projecting drawing data for a building onto a projection plane of the building under construction, the projection method comprising: measuring three or more distances, respectively, from three or more points on either of two straight lines (L1, L2) on the projection plane to the projection device using a distance meter included in the projection device (20, the two straight lines being reference lines orthogonal to each other and drawn on the projection plane in advance, the three or more points being not aligned in a straight line; measuring angles of the distance meter at a time at which the distances are measured, wherein the angles include a pan angle and a tilt angle; calculating a distance from the projection device to the projection plane and an inclination angle of the projection plane relative to the projection device, based on the distances measured and the angles measured; correcting distortion of the drawing data based on the calculated inclination angle; correcting a projection scaling factor for the drawing data based on the calculated distance; and projecting the drawing data onto a projection position on the projection plane, wherein, in the projecting, the drawing data on which the distortion correction is performed is projected onto the projection position using the corrected projection scaling factor; wherein the projection position is determined based on the distances measured and the angles of the distance meter at the time at which the distances are measured; wherein the drawing data includes position information of the two reference lines and the projection position is determined such that i) the two reference lines in the corrected drawing data overlap the two straight lines (L1, L2) on the projection plane, and ii) an intersection point of the two straight lines (L1, L2) overlaps a predetermined point of the corrected drawing data. measuring angles of the distance meter at a time at which the distances are measured, wherein the angles include a pan angle and a tilt angle; calculating a distance from the projection device to the projection plane and an inclination angle of the projection plane relative to the projection device, based on the distances measured and the angles measured; correcting distortion of the drawing data based on the calculated inclination angle; correcting a projection scaling factor for the drawing data based on the calculated distance; and projecting the drawing data onto a projection position on the projection plane, wherein, in the projecting, the drawing data on which the distortion correction is performed is projected onto the projection position using the corrected projection scaling factor; wherein the projection position is determined based on the distances measured and the angles of the distance meter at the time at which the distances are measured; wherein the drawing data includes position information of the two reference lines and the projection position is determined such that i) the two reference lines in the corrected drawing data overlap the two straight lines (L1, L2) on the projection plane, and ii) an intersection point of the two straight lines (L1, L2) overlaps a predetermined point of the corrected drawing data.