1. A projection image generating method of generating a projection image including information on at least a part of a first three-dimensional image and information on at least a part of a second three-dimensional image, comprising: a first accumulating step of accumulating plural first two-dimensional image data; a first three-dimensional imaging step of generating first three-dimensional image data from the plural first two-dimensional image data; a second accumulating step of accumulating plural second two-dimensional image data; a second three-dimensional imaging step of generating second three-dimensional image data from the plural second two-dimensional image data; a first projecting step of subjecting the first three-dimensional image data to rendering processing to generate a first projection image; and a second projecting step of subjecting the second three-dimensional image data to rendering processing to generate a second projection image, wherein the first projecting step generates the first projection image on the basis of the first three-dimensional image data and at least a part of the second three-dimensional image data, and the second projecting step generates the second projection image on the basis of the second three-dimensional image data and at least a part of the first three-dimensional image data, wherein the first projecting step corrects information of at least a partial area of the first projection image on the basis of the second three-dimensional image data, and the second projecting step corrects information of at least a partial area of the second projection image on the basis of the first three-dimensional image data.