A measurement system for measuring a distance from an object to be measured including a hole and fixed to a spindle of a machining apparatus, the measurement system comprising: a measuring instrument; a measurement control device; and a ranging device connected to the measurement control device, wherein the measuring instrument includes a base to be fixed to a tool shaft, a cutter holder, or a tailstock of the machining apparatus, a first rod connected to the base, a second rod connected to the base, a first measuring head fixed to the first rod and adapted to irradiate the object to be measured with measurement light emitted from an end of a first optical fiber connected to the ranging device, and a second measuring head fixed to the second rod and adapted to irradiate the object to be measured with measurement light emitted from an end of a second optical fiber connected to the ranging device, wherein each one of the first and second measuring heads comprises a lens for converting the light emitted from the end of the respective one of the first and second optical fibers into condensed light, a reflective mirror for bending the light from the lens, and a protection window adapted to transmit the light from the reflective mirror to a surface of the object to be measured and to return the light reflected or scattered from the surface to the respective one of the first and second optical fibers along the same optical path; wherein the ranging device includes a light source of the measurement light and is adapted to measure a distance to the object to be measured on the basis of reflected or scattered light of the measurement lights returning from the first and second measuring heads, and the measurement control device is adapted to execute a first operation for acquiring a distance I between the protection window of the first measuring head and an inner peripheral surface of the hole of the object to be measured from the ranging device, a second operation for acquiring a distance O between the protection window of the second measuring head and an outer peripheral surface of the object to be measured from the ranging device, and a third operation for calculating the thickness of the object to be measured on the basis of the distance I, the distance O, and a distance between the first and second measuring heads; wherein the ranging device is adapted to use a distance measurement method based on OCT, optical coherence tomography, or FMCW. frequency modulated continuous wave, or TOF, time of flight, or phase shift, or ranging using optical frequency combs for the first and second operations of the measurement control device. a measuring instrument; a measurement control device; and a ranging device connected to the measurement control device, wherein the measuring instrument includes a base to be fixed to a tool shaft, a cutter holder, or a tailstock of the machining apparatus, a first rod connected to the base, a second rod connected to the base, a first measuring head fixed to the first rod and adapted to irradiate the object to be measured with measurement light emitted from an end of a first optical fiber connected to the ranging device, and a second measuring head fixed to the second rod and adapted to irradiate the object to be measured with measurement light emitted from an end of a second optical fiber connected to the ranging device, wherein each one of the first and second measuring heads comprises a lens for converting the light emitted from the end of the respective one of the first and second optical fibers into condensed light, a reflective mirror for bending the light from the lens, and a protection window adapted to transmit the light from the reflective mirror to a surface of the object to be measured and to return the light reflected or scattered from the surface to the respective one of the first and second optical fibers along the same optical path; wherein the ranging device includes a light source of the measurement light and is adapted to measure a distance to the object to be measured on the basis of reflected or scattered light of the measurement lights returning from the first and second measuring heads, and the measurement control device is adapted to execute a first operation for acquiring a distance I between the protection window of the first measuring head and an inner peripheral surface of the hole of the object to be measured from the ranging device, a second operation for acquiring a distance O between the protection window of the second measuring head and an outer peripheral surface of the object to be measured from the ranging device, and a third operation for calculating the thickness of the object to be measured on the basis of the distance I, the distance O, and a distance between the first and second measuring heads; wherein the ranging device is adapted to use a distance measurement method based on OCT, optical coherence tomography, or FMCW. frequency modulated continuous wave, or TOF, time of flight, or phase shift, or ranging using optical frequency combs for the first and second operations of the measurement control device.