1. An image display method, comprising: a three-dimensional image creating step of creating a three-dimensional image based on volume data; 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, the positional relationship display image in which an opacity in the positional relationship display image of the cutting plane is adjusted so that a portion of the three-dimensional image that is hidden by the cutting plane is visually recognizable through the cutting plane in the positional relationship display image, wherein in the positional relationship display image creating step, an opacity in the positional relationship display image of the three-dimensional image is adjusted so that a portion of the cutting plane that is hidden by the three-dimensional image is visually recognizable through the three-dimensional image in the positional relationship display image.