An optical interferometer comprising a branching-combining unit; a first optical system; a second optical system; and a drive unit, wherein the branching-combining unit includes a branching surface, an incident surface, a first output surface, a combining surface, and a second output surface on an interface between the interior and the exterior of a transparent member, the branching surface and the combining surface are provided separately, the branching surface is configured to partially reflect incident light (L 0) entering from the outside and outputs as first branched light (L 11, L 12), and to transmit the rest of the incident light into the interior as second branched light (L 21, L 22), the incident surface is configured to transmit the first branched light entering from the branching surface via the first optical system into the interior, the first output surface is configured to output the second branched light reaching from the branching surface through the interior to the outside, the combining surface is configured to partially combine the first branched light reaching from the incident surface through the interior, and the second branched light entering from the first output surface via the second optical system to be output to the outside as first combined light (L 3), and to combine the rest of the first branched light and the second branched light to be propagated into the interior as second combined light (L 4, L 41, L 42), and the second output surface is configured to partially output the second combined light reaching from the combining surface through the interior to the outside, and the first optical system is configured to reflect the first branched light output from the branching surface by mirror (21,22), and to direct the light to the incident surface, the second optical system is configured to reflect the second branched light output from the first output surface by a mirror (31,32) and to direct the light to the combining surface, wherein the branching-combining unit further includes a third output surface on the interface between the interior and the exterior of the transparent member, and the third output surface is configured to output the rest of the second combined light reflected by the second output surface and reaching through the interior to the outside, the drive unit is configured to move the mirror (21, 22, 31, 32) of the first optical system or the second optical system to adjust an optical path difference between the first branched light and the second branched light from the branching surface to the combining surface, the optical interferometer further comprising: a first light receiving element configured to receive the first combined light output from the combining surface and to output a first current signal I 1 according to an amount of the received light; a second light receiving element configured to receive the second combined light partially output from the second output surface and to output a second current signal I 2 according to an amount of the received light; a third light receiving element configured to receive the rest of the second combined light output from the third output surface and to output a third current signal I 3 according to an amount of the received light; and a current-voltage conversion unit configured to output a voltage signal according to a value I 1 - I 2 - I 3 obtained by subtracting the second current signal I 2 and the third current signal I 3 from the first current signal I 1. a first light receiving element configured to receive the first combined light output from the combining surface and to output a first current signal I 1 according to an amount of the received light; a second light receiving element configured to receive the second combined light partially output from the second output surface and to output a second current signal I 2 according to an amount of the received light; a third light receiving element configured to receive the rest of the second combined light output from the third output surface and to output a third current signal I 3 according to an amount of the received light; and a current-voltage conversion unit configured to output a voltage signal according to a value I 1 - I 2 - I 3 obtained by subtracting the second current signal I 2 and the third current signal I 3 from the first current signal I 1.