A geometric correction adjustment method for adjusting a distortion incurred because a projection target region of a projection target object is not perpendicular to an optical axis of a projection apparatus configured to project a projection image on the projection target object, the projection apparatus including an output display element which has an element region including a plurality of pixels that modulate projection light, the element region including a quadrangular effective element region generating the projection image to be projected on said projection target object based on an input image, the geometric correction adjustment method comprising: providing for acquisition of adjustment instructions from a user to adjust the effective element region in the element region of the projection apparatus; projecting a projection image corresponding to the element region, such that the projection region of the projection image envelops a quadrangular projection target region of the projection target object; the method being characterized by in response to an adjustment instruction from the user, shifting a vertex (641, 642, 643, 644) of the effective element region along one side of two sides (646, 647, 648, 649) having an end that is the vertex (641, 642, 643, 644) to be shifted, to transform the effective element region, until a line in the projection region representing the other side of said two sides (646, 647, 648, 649) of the effective element region comes in contact with a vertex (721, 722, 723, 724) of the quadrangular projection target region of the projection target object corresponding to the vertex (641, 642, 643, 644) of the effective element region; in response to an adjustment instruction from the user, shifting the vertex (641, 642, 643, 644) of the effective element region along the other side of said two sides, to transform said effective element region, until the vertex of the projection region corresponding to said vertex of the effective element region (641, 642, 643, 644) is matched with the vertex (721, 722, 723, 724) of the quadrangular projection target region; repeating steps and for the other three vertexes of the effective element region. providing for acquisition of adjustment instructions from a user to adjust the effective element region in the element region of the projection apparatus; projecting a projection image corresponding to the element region, such that the projection region of the projection image envelops a quadrangular projection target region of the projection target object; the method being characterized by in response to an adjustment instruction from the user, shifting a vertex (641, 642, 643, 644) of the effective element region along one side of two sides (646, 647, 648, 649) having an end that is the vertex (641, 642, 643, 644) to be shifted, to transform the effective element region, until a line in the projection region representing the other side of said two sides (646, 647, 648, 649) of the effective element region comes in contact with a vertex (721, 722, 723, 724) of the quadrangular projection target region of the projection target object corresponding to the vertex (641, 642, 643, 644) of the effective element region; in response to an adjustment instruction from the user, shifting the vertex (641, 642, 643, 644) of the effective element region along the other side of said two sides, to transform said effective element region, until the vertex of the projection region corresponding to said vertex of the effective element region (641, 642, 643, 644) is matched with the vertex (721, 722, 723, 724) of the quadrangular projection target region; repeating steps and for the other three vertexes of the effective element region.