A passive optical network includes an optical line termination for generating multiple downward optical signals and multiple optical network units for receiving the downward optical signals. Each optical network unit includes a receiver for converting a corresponding downward optical signal inputted from the optical line termination into a first voltage signal and outputting the first voltage signal. Additionally, each optical network unit includes a transmitter for calculating a distance to the optical line termination from an intensity of the first voltage signal outputted from the receiver and generating an upward optical signal having an intensity adjusted according to the calculated distance to the optical line termination.