An oil state monitoring method for monitoring degradation states of oil used in machinery or equipment, comprising: a filtering step that filters the oil with a filter and captures oil oxidation products which are present in the oil on a surface of the filter and inside of the filter; a filter treatment step that removes the oil contents from the filter which filtered the oil in the filtering step and which captured oil oxidation products which are present in the oil prior to an oil filtration; a transmitted light-detection step that detects color components of transmitted light being light which is emitted onto the filter which captured the oil oxidation products treated in the filter treatment step and which penetrates through the filter inside of which the oil oxidation products are captured, wherein the transmitted light-detection step includes a step of detecting color components of first transmitted light which is first light that is emitted from a first light source (120,122) set on a side of the filter facing a first face of the filter which is the side where the oil is present prior to the oil filtering in the filtering step, that is emitted onto the filter treated in the filter treatment step, and that penetrates through the filter, to a side of the filter facing a second face which is on the reverse side of the filter with respect to the first face, a reflected light-detection step that detects, on the side of the first face, color components of first reflected light which is the first light reflected from the first face, a calculating step that calculates, using each color component indicated by RGB values of the first transmitted light and those of the first reflected light, a maximum color difference of both the first transmitted light and as well as a maximum color difference of the first reflected light, and in addition calculates a magnitude of a vector, in the RGB-three dimensional coordinates, of both the first transmitted light and as well as a magnitude of a vector in the RGB-three dimensional coordinates of the first reflected light; and a judging step that judges the oil degradation states using the calculated color difference of both the first transmitted light and that of the first reflected light and the calculated magnitudes of the vectors of both the first transmitted light and of the first reflected light, wherein the maximum color difference is defined as the difference between the greatest value and the least value in the RGB values of a given detected transmitted light or reflected light. a filtering step that filters the oil with a filter and captures oil oxidation products which are present in the oil on a surface of the filter and inside of the filter; a filter treatment step that removes the oil contents from the filter which filtered the oil in the filtering step and which captured oil oxidation products which are present in the oil prior to an oil filtration; a transmitted light-detection step that detects color components of transmitted light being light which is emitted onto the filter which captured the oil oxidation products treated in the filter treatment step and which penetrates through the filter inside of which the oil oxidation products are captured, wherein the transmitted light-detection step includes a step of detecting color components of first transmitted light which is first light that is emitted from a first light source (120,122) set on a side of the filter facing a first face of the filter which is the side where the oil is present prior to the oil filtering in the filtering step, that is emitted onto the filter treated in the filter treatment step, and that penetrates through the filter, to a side of the filter facing a second face which is on the reverse side of the filter with respect to the first face, a reflected light-detection step that detects, on the side of the first face, color components of first reflected light which is the first light reflected from the first face, a calculating step that calculates, using each color component indicated by RGB values of the first transmitted light and those of the first reflected light, a maximum color difference of both the first transmitted light and as well as a maximum color difference of the first reflected light, and in addition calculates a magnitude of a vector, in the RGB-three dimensional coordinates, of both the first transmitted light and as well as a magnitude of a vector in the RGB-three dimensional coordinates of the first reflected light; and a judging step that judges the oil degradation states using the calculated color difference of both the first transmitted light and that of the first reflected light and the calculated magnitudes of the vectors of both the first transmitted light and of the first reflected light, wherein the maximum color difference is defined as the difference between the greatest value and the least value in the RGB values of a given detected transmitted light or reflected light.