Distance measuring instrument

The invention provides a distance measuring instrument, comprising a light emitting unit 13 for emitting a distance measuring light, a photodetecting unit 14 for receiving and detecting a reflected distance measuring light from an object to be measured and a part of the distance measuring light emitted from the light emitting unit as an internal reference light, a sensitivity adjusting unit 23 for electrically adjusting photodetecting sensitivity of the photodetecting unit, and a control arithmetic unit 22 for calculating a measured distance based on a photodetection signal of the reflected distance measuring light from the photodetecting unit and based on a photodetection signal of the internal reference light, wherein the control arithmetic unit can measure a distance by selecting a prism mode measurement and a non-prism mode measurement, and controls so that photodetecting sensitivity of the photodetecting unit is changed by the sensitivity adjusting unit in response to the selected measurement mode.