An optical temperature sensor includes an light-emitting-side measurement unit to measure a first light intensity of a measuring beam and a second light intensity of a reference beam, and a light-receiving-side measurement unit to measure a third light intensity of a first reflected beam of the measuring beam and a fourth light intensity of a second reflected beam of the reference beam. The optical temperature sensor further includes a control unit to adjust at least one of a first control target value of the first light intensity and a second control target value of the second light intensity based on at least one of the third light intensity and the fourth light intensity. The control unit controls at least one of the first light intensity and the second light intensity based on the adjusted at least one of the first and second control target values.