A 3-dimensional measuring device that is configured to radiate a range-finding light to a measurement target, to measure a distance to the measurement target on the basis of an internal reference light beam and a reflection range-finding light, which is the range-finding light reflected from the measurement target, and to detect a radiation direction of the range-finding light to thereby acquire 3-dimensional data on the measurement target, the 3-dimensional measuring device comprising: a light source unit that is configured to emit the range-finding light; a projection optical system that is configured to radiate the range-finding light emitted from the light source unit to a range-finding optical axis; a scanning mirror that is provided to be rotatable about a rotating shaft in a state of being inclined with respect to a shaft center of the rotating shaft to radiate the range-finding light guided from the projection optical system within a plane crossing the rotating shaft in a rotary manner; a light-receiving optical system that is configured to receive the reflection range-finding light having been reflected from the measurement target and guided by the scanning mirror; a reference light optical system (24; 24A; 24B) that is provided in a range outside a measuring range, in which the measurement target is irradiated with the range-finding light; and a light receiving element that is configured to receive the reflection range-finding light and the internal reference light guided from the reference light optical system (24; 24A; 24B), characterized in that: the reference light optical system (24; 24A; 24B) is within a radiation range in which the range-finding light is radiated by the scanning mirror in a rotary manner, to receive and reflect the range-finding light, reflected from the scanning mirror, as the internal reference light, this reference light optical system (24; 24A; 24B) being capable of changing a light quantity of the reflected internal reference light. a light source unit that is configured to emit the range-finding light; a projection optical system that is configured to radiate the range-finding light emitted from the light source unit to a range-finding optical axis; a scanning mirror that is provided to be rotatable about a rotating shaft in a state of being inclined with respect to a shaft center of the rotating shaft to radiate the range-finding light guided from the projection optical system within a plane crossing the rotating shaft in a rotary manner; a light-receiving optical system that is configured to receive the reflection range-finding light having been reflected from the measurement target and guided by the scanning mirror; a reference light optical system (24; 24A; 24B) that is provided in a range outside a measuring range, in which the measurement target is irradiated with the range-finding light; and a light receiving element that is configured to receive the reflection range-finding light and the internal reference light guided from the reference light optical system (24; 24A; 24B), characterized in that: the reference light optical system (24; 24A; 24B) is within a radiation range in which the range-finding light is radiated by the scanning mirror in a rotary manner, to receive and reflect the range-finding light, reflected from the scanning mirror, as the internal reference light, this reference light optical system (24; 24A; 24B) being capable of changing a light quantity of the reflected internal reference light.