A captured image decision device includes a synchronizing signal extraction section extracting a vertical synchronizing signal and a horizontal synchronizing signal for use in displaying a captured image captured by a camera, a time measurement section measuring a pulse interval time period of the vertical synchronizing signal, a segment area setting section configured to divide a frame of the captured image displayed according to the pulse interval time period by a horizontal virtual line, thus setting a plurality of segment areas divided by the virtual line, a pulse number measurement section measuring a pulse number of the horizontal synchronizing signal corresponding to each segment area, and a decision section deciding whether an affirmation condition of the pulse number measured by the pulse number measurement section being equal to or greater than a predetermined value is satisfied or not.