Reproducing device and optical path length servo control method

A reproducing device comprising:

• a homodyne detection part configured to irradiate a recording layer of an optical recording medium with first light via an objective lens and irradiate a mirror with second light, the first light and the second light being obtained by splitting light emitted from a light source, the homodyne detection part carrying out homodyne detection by using reflected light of the first light obtained from the recording layer as signal light and using reflected light of the second light by the mirror as reference light;
• a signal reproducing part configured to obtain a reproduced signal based on the signal light, based on a result of homodyne detection by the homodyne detection part;
• a one-axis actuator configured to drive the mirror in a direction parallel to an incident optical axis of the second light to the mirror;
• an optical path length servo control part configured to carry out driving control of the one-axis actuator in such a manner that an optical path length difference between the signal light and the reference light is kept constant based on a light reception signal by a light receiver that receives the signal light and the reference light;
• and
• an offset giving part configured to give an offset that executes one of reduction and enlargement of an amplitude difference of the reproduced signal of marks that are recorded in the recording layer and have sizes different from each other, to an optical path length servo loop formed in association with servo control by the optical path length servo control part.