To provide an internal diameter measuring device capable of directly reading an internal diameter value.

When measuring a work, a work internal diameter value of an object hole is calculated by using a measured value (S4), and the calculated result is displayed on a display section as the internal diameter value (S5). In this instance, since the flow rate of compressed air passes through the object hole is proportional to the square measure of the object hole, it becomes proportional to the fourth power of the internal diameter value of the object hole, and a nonlinear error is produced in a region ranging from a calibration point calibrated in a small compass to a calibration point calibrated in a large compass. The internal diameter value, however, is calculated by using calculating formula adopting the fourth root of the measured value obtained by measuring a backing pressure, and the calculated internal diameter value is displayed on the display section, thereby directly reading the internal diameter value of the object hole.