A projection device comprising: storage means that stores at least a pattern image for detecting distortion depending on a surface state of a projection surface, an expected image that is an image which is expected to be displayed when the pattern image is projected and displayed on a flat surface, and one or more projection images to be projected on the projection surface, the storage means being configured to store the pattern image and the expected image both divided into a plurality of triangular areas associated with each other; image output means that outputs the pattern image acquired from the storage means; projection means that projects, on the projection surface, vertexes of the triangular areas constituting the pattern image output from the image output means successively; image acquisition means that acquires a displayed image of the pattern image displayed on the projection surface, and extracts the displayed image as a distortion image; distortion amount calculating means that calculates a distortion parameter relating to distortion of the projection surface by comparing the expected image with the distortion image with respect to each pair of the associated triangular areas; and distortion correction processing means that corrects the one or more projection images, by using the distortion parameter, wherein the image output means is configured to output, to the projection means, the one or more projection images corrected by the distortion correction processing means, the projection means is configured to project the corrected one or more projection images on the projection surface; and the distortion amount calculating means is configured to calculate, each time vertexes on the displayed image of the pattern image displayed on the projection surface form a triangle, the distortion parameter relating to a formed triangle successively. storage means that stores at least a pattern image for detecting distortion depending on a surface state of a projection surface, an expected image that is an image which is expected to be displayed when the pattern image is projected and displayed on a flat surface, and one or more projection images to be projected on the projection surface, the storage means being configured to store the pattern image and the expected image both divided into a plurality of triangular areas associated with each other; image output means that outputs the pattern image acquired from the storage means; projection means that projects, on the projection surface, vertexes of the triangular areas constituting the pattern image output from the image output means successively; image acquisition means that acquires a displayed image of the pattern image displayed on the projection surface, and extracts the displayed image as a distortion image; distortion amount calculating means that calculates a distortion parameter relating to distortion of the projection surface by comparing the expected image with the distortion image with respect to each pair of the associated triangular areas; and distortion correction processing means that corrects the one or more projection images, by using the distortion parameter, wherein the image output means is configured to output, to the projection means, the one or more projection images corrected by the distortion correction processing means, the projection means is configured to project the corrected one or more projection images on the projection surface; and the distortion amount calculating means is configured to calculate, each time vertexes on the displayed image of the pattern image displayed on the projection surface form a triangle, the distortion parameter relating to a formed triangle successively.