© 2018, The Optical Society of Japan. The diameter of cylindrical openings is conventionally measured with the mechanical and contact method. In this paper, we propose a contactless optical approach to measure dimensions of inner profile by a disk beam probe. The measurement is carried out illuminating the inner cylindrical opening with a light sheet generated by the disk beam probe, acquiring the light-section image of the object with an image sensor through a widefield lens, then numerically extracting the diameter. During measurement, the probe is aligned inside the openings with high freedom, which causes the optical section to manifest a variety of ellipses besides a circle accordingly. The measurement method copes with extracting the diameter of openings from various ellipse optical sections. The high freedom of probe alignment allows optical diameter measurement to be flexible and simple. The dimensions of cylindrical openings of manufactured parts were practically measured with the disk beam probe, and the results were confirmed with those measured with the vernier caliper.