An isolated synchronous rectifying DC/DC converter includes a drain terminal connected to a drain of a synchronous rectification transistor, a source terminal connected to a source of the synchronous rectification transistor; a comparator configured to compare a drain voltage of the drain terminal with a predetermined threshold voltage which set is based on a potential of the source terminal, a first flip-flop to which an OFF signal output from the comparator is input, a driver configured to output a gate signal to the synchronous rectification transistor based on an output signal of the first flip-flop, and a first abnormality detection circuit including an abnormality detection comparator configured to compare a voltage of the source terminal with a detection threshold voltage, and configured to output a first abnormality detection signal based on an output of the abnormality detection comparator.