Apparatus for determining the effective case-hardening or nitriding depth of a steel gear to be measured, comprising: - an evolventimeter suitable for identifying a portion of said gear to be measured and of a reference gear of the non case-hardened or non-nitrided type; and - a measuring head connected to the evolventimeter and including a laser source configured to generate a variable frequency radiation for scanning portions of the gear to be measured and of the reference gear identified by the evolventimeter; and wherein said measuring head furthermore comprises: - an infrared detector configured to detect infrared radiation generated by the gear to be measured and by the reference gear in response to scanning by the laser source; - first processing means configured to determine, on the basis of the infrared radiation detected, at least one between a first and a second measured spectrum, the first measured spectrum indicating the difference between the phase of the infrared radiation generated by the gear to be measured and the phase of the infrared radiation generated by the reference gear, the second measured spectrum indicating the difference between the amplitude of the infrared radiation generated by the gear to be measured and the amplitude of the infrared radiation generated by the reference gear; - first computing means (26, 40-56) configured to compute a calculated hardness profile of the gear to be measured, on the basis of a launch profile equal to a first hardness profile, and of said at least one between the first and the second measured spectrum; and - second computing means (26, 58) configured to compute the effective case-hardening or nitriding depth, on the basis of the calculated hardness profile. - an evolventimeter suitable for identifying a portion of said gear to be measured and of a reference gear of the non case-hardened or non-nitrided type; and - a measuring head connected to the evolventimeter and including a laser source configured to generate a variable frequency radiation for scanning portions of the gear to be measured and of the reference gear identified by the evolventimeter; - an infrared detector configured to detect infrared radiation generated by the gear to be measured and by the reference gear in response to scanning by the laser source; - first processing means configured to determine, on the basis of the infrared radiation detected, at least one between a first and a second measured spectrum, the first measured spectrum indicating the difference between the phase of the infrared radiation generated by the gear to be measured and the phase of the infrared radiation generated by the reference gear, the second measured spectrum indicating the difference between the amplitude of the infrared radiation generated by the gear to be measured and the amplitude of the infrared radiation generated by the reference gear; - first computing means (26, 40-56) configured to compute a calculated hardness profile of the gear to be measured, on the basis of a launch profile equal to a first hardness profile, and of said at least one between the first and the second measured spectrum; and - second computing means (26, 58) configured to compute the effective case-hardening or nitriding depth, on the basis of the calculated hardness profile.