A lighting circuit (1) includes a controller integrated circuit (U1) for on and off control of a switching element (Q1), a transformer (T1) having a primary coil (T11) that is connected in series to the switching element and a secondary coil (T12), a first voltage supply circuit supplying a voltage that occurs across both ends of the primary coil to an LED module (2), and a second voltage supply circuit supplying a voltage induced in the secondary coil to a power source terminal (Vcc) when the switching element is on. A turns ratio N of the primary coil to the secondary coil satisfies Equation 1, where X1 V denotes a voltage that an AC power source supplies, Y V denotes the maximum value of a power source voltage of the control unit at which normal operation is guaranteed, and Z V denotes the minimum value of the power source voltage.