A light emitting device includes a light emitting unit and a constant current controller. The light emitting unit includes at least one LED string. The constant current controller is connected to a cathode terminal of the LED string and controls a driving current flowing through the LED string. The constant current controller includes a transistor connected to the cathode terminal of the LED string. A switching unit and a sensing resistor are connected to the transistor. The switching unit connects and disconnects between the sensing resistor and the transistor.