A switched-mode power supply apparatus, comprising: A transformer, which comprises: a core having a linear center leg portion in the center portion thereof; a primary winding provided around the linear center leg portion of the core; at least two secondary windings (12, 13) provided around the linear center leg portion of the core and having a winding axis which is the same as a winding axis of the primary winding, which are configured to be connected to a load and supply electric power, which is supplied by the primary winding, to the load; wherein at least two auxiliary windings (14, 16) are provided around the linear center leg portion of the core and have a winding axis which is the same as the winding axis of the primary winding, wherein the auxiliary windings (14, 16) respectively neighbor the secondary windings (12, 13), and are connected in parallel to each other, and; wherein the secondary windings (12, 13) are disposed at both sides of the primary winding in the winding axis direction of the primary winding and are disposed closer to the primary winding than the auxiliary windings (14, 16), in the winding axis direction of the primary winding; characterized in that a switching element is electrically connected to the primary winding of the transformer to control an input voltage, which is supplied by an external power source into the primary winding; and a control circuit configured to control the switching element, wherein the auxiliary windings (14, 16) of the transformer are electrically connected with the control circuit and provide the control circuit with electric power, which is supplied by the primary winding, for driving the switching element. A transformer, which comprises: a core having a linear center leg portion in the center portion thereof; a primary winding provided around the linear center leg portion of the core; at least two secondary windings (12, 13) provided around the linear center leg portion of the core and having a winding axis which is the same as a winding axis of the primary winding, which are configured to be connected to a load and supply electric power, which is supplied by the primary winding, to the load; a core having a linear center leg portion in the center portion thereof; a primary winding provided around the linear center leg portion of the core; at least two secondary windings (12, 13) provided around the linear center leg portion of the core and having a winding axis which is the same as a winding axis of the primary winding, which are configured to be connected to a load and supply electric power, which is supplied by the primary winding, to the load; wherein at least two auxiliary windings (14, 16) are provided around the linear center leg portion of the core and have a winding axis which is the same as the winding axis of the primary winding, wherein the auxiliary windings (14, 16) respectively neighbor the secondary windings (12, 13), and are connected in parallel to each other, and; wherein the secondary windings (12, 13) are disposed at both sides of the primary winding in the winding axis direction of the primary winding and are disposed closer to the primary winding than the auxiliary windings (14, 16), in the winding axis direction of the primary winding; characterized in that a switching element is electrically connected to the primary winding of the transformer to control an input voltage, which is supplied by an external power source into the primary winding; and wherein the secondary windings (12, 13) are disposed at both sides of the primary winding in the winding axis direction of the primary winding and are disposed closer to the primary winding than the auxiliary windings (14, 16), in the winding axis direction of the primary winding; characterized in that a switching element is electrically connected to the primary winding of the transformer to control an input voltage, which is supplied by an external power source into the primary winding; and a control circuit configured to control the switching element, wherein the auxiliary windings (14, 16) of the transformer are electrically connected with the control circuit and provide the control circuit with electric power, which is supplied by the primary winding, for driving the switching element.