1. In a machining simulation which displays a shape of a material, a shape of a product, and a shape of a tool as images on a display unit, and moves the displayed image of the tool on the display unit on the basis of previously generated tool path data, and eliminates portions of the image of the material and of the image of the product which overlap the image of the tool in order, a method of displaying the shape of the material and the shape of the product in the machining simulation comprising following steps: displaying the shape of the product in accordance with a portion to be machined as a line image indicating only an outline of the product on the basis of dimension data of the product, and displaying the shape of the material in accordance with the portion to be machined as a plane image indicating an area in correspondence with a machining allowance on the basis of machining allowance data, in case that the material includes the portion to be machined which is formed by adding the specified machining allowance to the outline of the product.