PURPOSE: To aim at output improvements in a DC-DC converter with no regulation as well as to control it into optimum efficiency all the time, by regulating an 'on' time in a power switching element according to primary voltage of the DC-DC converter available for an internal-combustion engine ignition device.

CONSTITUTION: The flyback voltage produced by switching of a power transistor 16 is inputted into a converter 28, then impressed on each of differentiation circuits 22, 29 and 30, producing a differentiation pulse there, and further impressed on the base of a transistor 31. With this, the base impressing pulse of a transistor 15 becomes shortened. Since collector voltage of the power transistor 16 has set a flat part in advance, this setting is of nonregulation and also an 'on' time control circuit 32 comes into nonregulation. With this constitution, a DC-DC converter 20 is automatically set in optimum efficiency whereby 'on' time in the power transistor 16 is amply prolonged, thus output voltage is improved as well.