To obtain a theodolite facing mirror used for the calibration between two theodolites of a main theodolite and an auxiliary theodolite and a calibrating method using the mirror.

At the time of finding the position and angle of an auxiliary theodolite 3 which is mounted on an appropriate position so that the theodolite 3 can rotate around its mounting center 301 relatively to a main theodolite 2 by aligning the optical axes 4 of the theodolites 2 and 3 with each other, a theodolite facing mirror 1 is fixed to the theodolite 3 by putting the female screw tap 1-4 of the mirror 1 on a male screw tap 3-2 at the end section of the theodolite 3. Then the optical axis 4 of the main theodolite 2 is fixed toward the mirror 1 mounted on the auxiliary theodolite 3 and automatically collimated. Consequently, the optical axes 4 of the theodolites 2 and 3 are completely aligned with each other.