A step-up and step-down DC-DC converter comprising: a step-down unit that has a step-down switch inputting a direct-current voltage from a direct-current power supply and a choke coil connected to the step-down switch, and that outputs an output voltage lower than the input voltage to a light source; a step-up unit that has a step-up switch connected to the step-down switch via the choke coil, and that outputs an output voltage higher than the input voltage to the light source; a constant-current clamping circuit that limits a light source driving current flowing through the light source to a constant value; and a control unit that controls the step-down switch, the step-up switch and the constant-current clamping circuit on the basis of the light source driving current, characterized in that: the control unit includes a mode switching control unit that switches a mode to any one of a step-down mode in which the step-down switch is subjected to switching and the step-up switch maintains an off state, a conductive mode in which the step-down switch maintains an on state and the step-up switch maintains an off state and a step-up mode in which the step-down switch maintains an on state and the step-up switch is subjected to switching, wherein a maximum value of an on-duty of the step-down switch and a minimum value of an on-duty of the step-up switch are preset in the control unit, the constant-current clamping circuit starts limiting operation that controls the light source driving current to a constant value when the on-duty of the step-down switch becomes the maximum value in the step-down mode or when the on-duty of the step-up switch becomes the minimum value in the step-up mode, and the constant-current clamping circuit stops the limiting operation when the on-duty of the step-down switch becomes smaller than the maximum value in the step-down mode or when the on-duty of the step-up switch becomes larger than the minimum value in the step-up mode. a step-down unit that has a step-down switch inputting a direct-current voltage from a direct-current power supply and a choke coil connected to the step-down switch, and that outputs an output voltage lower than the input voltage to a light source; a step-up unit that has a step-up switch connected to the step-down switch via the choke coil, and that outputs an output voltage higher than the input voltage to the light source; a constant-current clamping circuit that limits a light source driving current flowing through the light source to a constant value; and a control unit that controls the step-down switch, the step-up switch and the constant-current clamping circuit on the basis of the light source driving current, characterized in that: the control unit includes a mode switching control unit that switches a mode to any one of a step-down mode in which the step-down switch is subjected to switching and the step-up switch maintains an off state, a conductive mode in which the step-down switch maintains an on state and the step-up switch maintains an off state and a step-up mode in which the step-down switch maintains an on state and the step-up switch is subjected to switching, wherein a maximum value of an on-duty of the step-down switch and a minimum value of an on-duty of the step-up switch are preset in the control unit, the constant-current clamping circuit starts limiting operation that controls the light source driving current to a constant value when the on-duty of the step-down switch becomes the maximum value in the step-down mode or when the on-duty of the step-up switch becomes the minimum value in the step-up mode, and the constant-current clamping circuit stops the limiting operation when the on-duty of the step-down switch becomes smaller than the maximum value in the step-down mode or when the on-duty of the step-up switch becomes larger than the minimum value in the step-up mode. the control unit includes a mode switching control unit that switches a mode to any one of a step-down mode in which the step-down switch is subjected to switching and the step-up switch maintains an off state, a conductive mode in which the step-down switch maintains an on state and the step-up switch maintains an off state and a step-up mode in which the step-down switch maintains an on state and the step-up switch is subjected to switching, wherein a maximum value of an on-duty of the step-down switch and a minimum value of an on-duty of the step-up switch are preset in the control unit, the constant-current clamping circuit starts limiting operation that controls the light source driving current to a constant value when the on-duty of the step-down switch becomes the maximum value in the step-down mode or when the on-duty of the step-up switch becomes the minimum value in the step-up mode, and the constant-current clamping circuit stops the limiting operation when the on-duty of the step-down switch becomes smaller than the maximum value in the step-down mode or when the on-duty of the step-up switch becomes larger than the minimum value in the step-up mode.