An oil deterioration detection device performs an analysis based on impedances at at least three first specific frequencies selected from a first frequency range to calculate an arc of impedance on a Nyquist diagram, the impedances being measured by applying an AC voltage at the at least three first specific frequencies to counter electrodes when oil to be evaluated passes between the counter electrodes, and when the calculated arc is larger than a reference arc calculated in advance of the oil to be evaluated by a predetermined value or larger, determines that moisture is mixed and calculates the amount of moisture based on the shape of the calculated arc.