A front photo detector (FPD) of an optical pick-up for an optical recording and playing apparatus having no gain selection switch, a simplified configuration, an improved prevention of an abnormal operation due to an error in gain selection, and improved reliability. The front photo detector includes a photo diode unit, a current-voltage amplifying circuit, and a voltage amplifying unit. The photo diode unit outputs current proportional to the power of the light emitted from one of a plurality of laser diodes. The current-voltage amplifying circuit converts the current output from the photo diode unit into a voltage. The voltage amplifying circuit amplifies the voltage output from the current-voltage amplifying circuit by a predetermined gain.