To prevent fluctuation up and down and left and right in splitting work by a splitting tool useful for splitting a slender tube such as a medical catheter by providing a center guide pin till reaching a razor blade in a frame from a tube inserting hole at one end of the frame.

Observation of the inner surface of a slender tube requires splitting of the tube into two, and a splitting tool is used for this splitting. This splitting tool consists of an end part 31 with a bored tube inserting hole 4 and forked parts 32 extending in the direction of tube passing through from this end part 31, and has a frame 3 in which a space is formed between the forked parts 32. Razor blades are provided at an appropriate place of the space of the frame 3. At this time, a center guide pin 2 is provided so as to be inserted through the space between the forked parts 32 of the frame 3, and two razor blades 1 are set on the center guide pin 2 symmetrically.