1. An image display method, comprising: a three-dimensional image creating step of creating a three-dimensional image based on volume data by a maximum intensity projection of the volume data from a viewpoint to a projection plane, wherein for each of pixels forming the three-dimensional image, a maximum intensity voxel having a maximum intensity among a string of the volume data on a line of vision that passes through the volume data from the viewpoint and reaches the pixel on the projection plane is determined, and an intensity and a Z coordinate on a Z axis along the line of view, of the maximum intensity voxel are taken respectively as an intensity and a Z coordinate of the pixel; a cutting plane setting step of setting a cutting plane which cuts the three-dimensional image at an arbitrary position; and a positional relationship display image creating step of creating a positional relationship display image which represents a mutual positional relationship between the three-dimensional image and the cutting plane, wherein in the positional relationship display image, an opacity of a first portion of the cutting plane is adjusted so that a first portion of the three-dimensional image that is hidden by the first portion of the cutting plane is visually recognizable through the first portion of the cutting plane while the first portion of the cutting plane is visually recognizable over the first portion of the three-dimensional image in the positional relationship display image, and an opacity of a second portion of the three-dimensional image is adjusted so that a second portion of the cutting plane that is hidden by the second portion of the three-dimensional image is visually recognizable through the second portion of the three-dimensional image while the second portion of the three-dimensional image is visually recognizable over the second portion of the cutting plane in the positional relationship display image, wherein in the positional relationship display image creating step, the mutual positional relationship between the three-dimensional image and the cutting plane in relation to the viewpoint is determined in accordance with the Z coordinate of each of the pixels forming the three-dimensional image and a Z coordinate on the Z axis of the cutting plane, when the three-dimensional image is closer to the viewpoint than the cutting plane, an intensity of the positional relationship display image is calculated as a combination of an intensity of the three-dimensional image and an opacity of the three-dimensional image added to a combination of a transparency of the three-dimensional image, an intensity of the cutting plane and an opacity of the cutting plane, and when the cutting plane is closer to the viewpoint than the three-dimensional image, the intensity of the positional relationship display image is calculated as a combination of the intensity of the cutting plane, the opacity of the cutting plane added to a combination of a transparency of the cutting plane, the intensity of the three-dimensional image and the opacity of the three-dimensional image.