Inner surface measuring apparatus

The inner surface shape of a hole, in particular, the inner surface shape of a hole the entry of which is narrow and which becomes wider further in from the entry, can be measured at a high level of accuracy. An inner surface measuring apparatus is provided which comprises: a low coherent light source that outputs low coherent light in to two; a low coherent light dividing section that divides one portion of the low coherent light; a light path length adjusting section that adjusts a light path length of one of the low coherent lights divided by the low coherent light dividing section; a straight rod shaped probe that irradiates the other of the low coherent lights divided by the low coherent light dividing section from a front end section onto a measurement object arranged in a direction intersecting with the lengthwise direction of the probe; a low coherent light multiplexing section that multiplexes a low coherent light that returns having been reflected on the measurement object with the one low coherent light whose light path length has been adjusted by the light path length adjusting section; a light detector that detects the multiplexed low coherent light; and a rotation mechanism that relatively rotates the probe and the measurement object about an axis along the lengthwise direction of the probe.