A lighting device includes a step-down chopper circuit and a controller configured to control the step-down chopper circuit. The controller is configured to turn on and off a low-side switching device in the step-down chopper circuit at a fixed frequency. The controller includes an operation unit configured to determine a reference voltage value, an output unit configured to output, as a difference signal, a difference between the reference voltage value determined by the operation unit and an average value of a voltage proportional to a current flowing through the switching device, and a control circuit configured to determine an on-time period of the switching device. The control circuit includes an oscillator configured to generate a voltage signal shaped like teeth of a saw. The control circuit is configured to determine the on-time period of the switching device based on the voltage signal and the difference signal.