When a detection voltage VCS that corresponds to a voltage drop across a detection resistor RCS exceeds a first threshold value VADIM, a current limit comparator asserts a reset pulse. A zero current detection circuit asserts a set pulse configured as an instruction to turn on a switching transistor. A logic circuit generates a control pulse based on the reset pulse and the set pulse. When a transition occurs in a dimming pulse from an on level to an off level in a period in which the control pulse is set to the on level, the dimming pulse adjusting unit delays this transition to a next off period of the control pulse.