PROBLEM TO BE SOLVED: To provide a nozzle diameter measuring gauge, a nozzle diameter measuring device, and a nozzle diameter measuring method capable of easily discriminating a nozzle diameter. An insertion unit (3) that can be inserted into a nozzle N to be measured is provided, and the insertion unit (3) includes a plurality of measurement units (30) whose external dimensions are gradually increased from the tip end side to the base end side thereof. Therefore, when the insertion portion 3 is inserted into the nozzle N, the end face of the nozzle N comes into contact with the measuring portion 30c having an external dimension larger than the diameter D of the nozzle N, and the insertion of the insertion portion 3 is stopped. Since the external dimensions of the measuring unit 30 are set to a value proportional to the dimensions from the tip of the insertion unit 3 to the base end of the measuring unit 30, the insertion unit until the insertion of the insertion unit 3 is stopped is stopped. By measuring the insertion depth of 3, the external dimension R of the measuring unit 30b inserted into the nozzle N in such a stopped state can be calculated. Since the calculated external dimension R is the nozzle diameter D, the diameter D can be easily determined even if the nozzle N has a small diameter. [Selection diagram] Fig. 1
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