Disclosed are an oil state monitoring method and an oil state monitoring device which monitor the state of degradation of oil used in machinery or equipment. In monitoring the oil state by the oil state monitoring method and by the oil state monitoring device, oil used in machinery or equipment is filtered when the degradation state of the aforementioned oil is to be monitored. By means of filtration, the oil content is removed from the filter which captured contaminants that were present in oil prior to filtration. Light is projected onto the filter from which oil was removed. The projected light detects the color components of the transmitted light which penetrated the aforementioned filter.