An oil-degradation detecting apparatus that can more accurately judge oil degradation and a mechanical system having a rotating part or a sliding part and including the oil-degradation detecting apparatus are provided. Two plates (21, 22) are disposed in an oil flow path (11) so as to be parallel to each other, an ammeter (24) measures a current that flows when an AC voltage is applied between the two plates (21, 22), a voltmeter measures the voltage between the plates (21, 22), and a signal processor (processor) (31) determines the electrical conductivity and the dielectric constant of the oil (10) based on the measurement results from the ammeter (24) and the voltmeter (25) and judges degradation of the oil (10) based on the electrical conductivity and the dielectric constant.