Recently, a one-pass type boring machine capable of completing the grinding process with only one stroke has been put into practical use. Realization of high-accuracy machining in this process requires appropriate management of the diameter of the abrasive tool to be used. However, because the form and adhesion posture of the electrodeposited abrasive grain are not uniform, measurements of the diameter may differ depending upon the point at which measurements are obtained. Therefore, even if abrasive tools with the same nominal diameter are used, there may be some differences in the dimensions of the machined hole. Using a non-contact measuring instrument that makes use of a transmitted light beam, we propose a method for determination of the effective tool diameter covering the whole cutting edge zone of the tool. Experiments showed that the proposed method is acceptable.