A power supply device comprising: a semiconductor light-emitting element; current detection means for detecting a current supplied to the semiconductor light-emitting element to output a current detection signal; voltage detection means for detecting a voltage applied to the semiconductor light-emitting element to output a voltage detection signal; reference signal output means for receiving a dimming signal indicative of a dimming level, to output a first reference signal (Vref1) to be compared with the current detection signal and a second reference signal (Vref2) to be compared with the voltage detection signal according to the dimming level; a dimming control means configured to be set in one of a current control mode and a voltage control mode according to the larger one of the first or second reference signals respectively, to output to a control means in the current control mode, a result of comparison between the current detection signal and the first reference signal which is higher than the second reference signal in the current control mode, and to output to the control means, in the voltage control mode, a result of comparison between the voltage detection signal and the second reference signal which is higher than the first reference signal in the voltage control mode; the control means configured to control, (i) in the current control mode, a direct electrical current continuously supplied to the semiconductor light-emitting element to drive the semiconductor light-emitting element according to the result of comparison supplied by the dimming control means, and (ii) in the voltage control mode, a load voltage applied to the semiconductor light-emitting element (19,21) which is continuously supplied with the direct electrical current to drive the semiconductor light emitting element according to the result of comparison supplied by the dimming control means. a semiconductor light-emitting element; current detection means for detecting a current supplied to the semiconductor light-emitting element to output a current detection signal; voltage detection means for detecting a voltage applied to the semiconductor light-emitting element to output a voltage detection signal; reference signal output means for receiving a dimming signal indicative of a dimming level, to output a first reference signal (Vref1) to be compared with the current detection signal and a second reference signal (Vref2) to be compared with the voltage detection signal according to the dimming level; a dimming control means configured to be set in one of a current control mode and a voltage control mode according to the larger one of the first or second reference signals respectively, to output to a control means in the current control mode, a result of comparison between the current detection signal and the first reference signal which is higher than the second reference signal in the current control mode, and to output to the control means, in the voltage control mode, a result of comparison between the voltage detection signal and the second reference signal which is higher than the first reference signal in the voltage control mode; the control means configured to control, (i) in the current control mode, a direct electrical current continuously supplied to the semiconductor light-emitting element to drive the semiconductor light-emitting element according to the result of comparison supplied by the dimming control means, and (ii) in the voltage control mode, a load voltage applied to the semiconductor light-emitting element (19,21) which is continuously supplied with the direct electrical current to drive the semiconductor light emitting element according to the result of comparison supplied by the dimming control means. (i) in the current control mode, a direct electrical current continuously supplied to the semiconductor light-emitting element to drive the semiconductor light-emitting element according to the result of comparison supplied by the dimming control means, and (ii) in the voltage control mode, a load voltage applied to the semiconductor light-emitting element (19,21) which is continuously supplied with the direct electrical current to drive the semiconductor light emitting element according to the result of comparison supplied by the dimming control means.