A switching power-supply device includes an input power detecting part that finds a DC input power from a detected DC input voltage and a detected DC input current, and an operation mode setting part that sets a burst mode operation or a continuous mode operation based on the DC power detected by the input power detecting part. The operation mode setting part also may set a ratio of a switching active period Tact, in which the main switching element is caused to perform switching operations, to a switching stop period Tstop, in which the switching operations are stopped, in the case of the burst mode operation in accordance with the detected input power.