Clearance measuring method for measuring the clearance between facing surfaces of a first member and a second member, wherein the surface of the first member is a circular surface of a cam of a cam shaft, and wherein the surface of the second member is circular or horizontal, the method comprising the steps of: bringing a pair of probes each having a round tip end (14, 611) slightly larger than the clearance into direct contact with the facing surfaces by means of moving means and drive means, wherein the moving means can move vertically and moves the pair of probes to a position of the clearance, wherein the drive means is connected to the pair of probes and is adapted to open and close the probes, wherein the pair of probes is united with a bracket which is seated on pins via respective surfaces, and wherein the pins are connected to the moving means, wherein the bracket has through holes extending through the bracket in a vertical direction and the pins are inserted through the respective through holes via respective compression springs urging the bracket upward in the vertical direction; moving the moving means downward in a vertical direction; gradually closing the pair of probes by means of the drive means in such a manner as to follow the profile of the first member until the tip ends (14, 611) come into contact with the valve clearance, and moving the moving means downward in the vertical direction further, wherein the bracket is not moved further, to separate the pins from the respective surfaces; and pushing the bracket (751 upwards with respect to the pins via the compression springs to enable a floating action; searching for the position of the clearance with the tip ends (14, 611); measuring the separation distance between the two tip ends (14, 611), and calculating and measuring the clearance value on the basis of the separation distance between the tip ends (14, 611). bringing a pair of probes each having a round tip end (14, 611) slightly larger than the clearance into direct contact with the facing surfaces by means of moving means and drive means, wherein the moving means can move vertically and moves the pair of probes to a position of the clearance, wherein the drive means is connected to the pair of probes and is adapted to open and close the probes, wherein the pair of probes is united with a bracket which is seated on pins via respective surfaces, and wherein the pins are connected to the moving means, wherein the bracket has through holes extending through the bracket in a vertical direction and the pins are inserted through the respective through holes via respective compression springs urging the bracket upward in the vertical direction; moving the moving means downward in a vertical direction; gradually closing the pair of probes by means of the drive means in such a manner as to follow the profile of the first member until the tip ends (14, 611) come into contact with the valve clearance, and moving the moving means downward in the vertical direction further, wherein the bracket is not moved further, to separate the pins from the respective surfaces; and pushing the bracket (751 upwards with respect to the pins via the compression springs to enable a floating action; searching for the position of the clearance with the tip ends (14, 611); measuring the separation distance between the two tip ends (14, 611), and calculating and measuring the clearance value on the basis of the separation distance between the tip ends (14, 611).