In recent years, there has been a great demand for large-diameter wafers with high flatness of hard and brittle materials that are used as optical and sensor elements. To meet these demands, we have developed an ultra-precision plane honing method as a highly efficient surface finishing technique using a fixed abrasive. This technique offers good finish surface roughness, shallow subsurface damage and high machining efficiency. However, there is a need to improve the surface flatness, which is dependent on the grinding wheel surface flatness and the tilt of the spindles. In this paper, the relationship between grinding wheel surface flatness and the shape of a truer is investigated by calculating the contact length of a point on the grinding wheel with the truer. It was found that there is an optimum shape of the truer to make the grinding wheel surface flat, and the machined 3-inch glass wafer is controlled to a flatness of less than 1µm by using the optimum truer.