A semiconductor light emitting element drive device, comprising: a direct-current power supply circuit (1, 2 and 3) configured to output direct-current power; and a converter circuit comprising a first switching device and a transformer, the converter circuit being configured to input the power outputted from the direct-current power supply circuit (1, 2, 3) to variably control an ON width of the first switching device, to thereby cause the converter circuit to supply a load current to a light source unit comprising light emitting elements; characterized in that the semiconductor light emitting element drive device further comprises; a current regulation circuit comprising a series circuit, of an impedance component (R19, R20) and a second switching device, connected in parallel with the light source unit, said current regulation circuit being configured to variably control the load current through the light source unit by regulation of an ON width of the second switching device; and a control unit configured to control ON and OFF of the first and second switching devices (Q1 and Q2), wherein the control unit is configured, (a) if a dimming ratio corresponding to a light output of the light source unit is higher than a first ratio, to increase the ON width of the first switching device with the second switching device kept off when increasing the dimming ratio corresponding to the light output of the light source unit, and also to decrease the ON width of the first switching device with the second switching device kept off when decreasing the dimming ratio corresponding to the light output of the light source unit, and also (b) if a dimming ratio corresponding to a light output of the light source unit is lower than the first ratio, to regulate the ON width of the second switching device with the ON width of the first switching device held at a lower limit, thereby controlling the dimming ratio corresponding to the light output of the light source unit. a direct-current power supply circuit (1, 2 and 3) configured to output direct-current power; and a converter circuit comprising a first switching device and a transformer, the converter circuit being configured to input the power outputted from the direct-current power supply circuit (1, 2, 3) to variably control an ON width of the first switching device, to thereby cause the converter circuit to supply a load current to a light source unit comprising light emitting elements; characterized in that the semiconductor light emitting element drive device further comprises; a current regulation circuit comprising a series circuit, of an impedance component (R19, R20) and a second switching device, connected in parallel with the light source unit, said current regulation circuit being configured to variably control the load current through the light source unit by regulation of an ON width of the second switching device; and a control unit configured to control ON and OFF of the first and second switching devices (Q1 and Q2), wherein the control unit is configured, (a) if a dimming ratio corresponding to a light output of the light source unit is higher than a first ratio, to increase the ON width of the first switching device with the second switching device kept off when increasing the dimming ratio corresponding to the light output of the light source unit, and also to decrease the ON width of the first switching device with the second switching device kept off when decreasing the dimming ratio corresponding to the light output of the light source unit, and also (b) if a dimming ratio corresponding to a light output of the light source unit is lower than the first ratio, to regulate the ON width of the second switching device with the ON width of the first switching device held at a lower limit, (a) if a dimming ratio corresponding to a light output of the light source unit is higher than a first ratio, to increase the ON width of the first switching device with the second switching device kept off when increasing the dimming ratio corresponding to the light output of the light source unit, and also to decrease the ON width of the first switching device with the second switching device kept off when decreasing the dimming ratio corresponding to the light output of the light source unit, and also (b) if a dimming ratio corresponding to a light output of the light source unit is lower than the first ratio, to regulate the ON width of the second switching device with the ON width of the first switching device held at a lower limit, thereby controlling the dimming ratio corresponding to the light output of the light source unit.