An apparatus for measuring a bio signal including a bio signal measurement unit, which is insertable into an ear to be in close contact with an internal surface of the ear, the bio signal measurement unit having a photo plethysmography (PPG) measurement module for radiating light of different wavelengths onto the internal surface of the ear, detecting light transmitted through the ear, and outputting a PPG signal including bio information, a control unit having a PPG signal processor for generating the bio information using the PPG signal measured by the PPG measurement module, and an output unit for displaying the bio information generated from the control unit. The apparatus preferably includes a temperature measurement module for measuring body temperature using infrared rays radiated from an internal surface of the ear. The output unit may be implemented by a liquid crystal display of a mobile phone.