A hydraulic circuit of a hydraulic working machine with a contamination level determination system for hydraulic oil, said system being adapted to sense foreign matter in hydraulic oil flowing through the hydraulic circuit of the hydraulic working machine and to determine a contamination level of the hydraulic oil by using results of the sensing, the contamination level determination system comprising: an onboard controller unit and characterized by comprising: a pilot filter arranged in a pilot line of the hydraulic circuit to remove, from the hydraulic oil, foreign matter that may cause an operation failure of the hydraulic working machine, a first light-blocking sensor for sensing passage of foreign matter on an upstream side of the filter, a second light-blocking sensor for sensing passage of foreign matter on a downstream side of the filter, wherein the onboard controller unit is designed to function as: a first frequency determination means for determining a frequency of sensing of foreign matter by the first light-blocking sensor, a second frequency determination means for determining a frequency of sensing of foreign matter by the second light-blocking sensor, a contamination level determination means for determining a contamination level on the upstream side of the filter on a basis of both results of determination by the first and second, frequency determination means, wherein the contamination level determination means is for correcting effect of miss-sensing on the first light-blocking sensor by comparing a result of determination of the sensing frequency of foreign matter of particle sizes by the first frequency determination means with a result of determination of the sensing frequency of foreign matter of the same particle sizes by the second frequency determination means. an onboard controller unit and characterized by comprising: a pilot filter arranged in a pilot line of the hydraulic circuit to remove, from the hydraulic oil, foreign matter that may cause an operation failure of the hydraulic working machine, a first light-blocking sensor for sensing passage of foreign matter on an upstream side of the filter, a second light-blocking sensor for sensing passage of foreign matter on a downstream side of the filter, wherein the onboard controller unit is designed to function as: a first frequency determination means for determining a frequency of sensing of foreign matter by the first light-blocking sensor, a second frequency determination means for determining a frequency of sensing of foreign matter by the second light-blocking sensor, a contamination level determination means for determining a contamination level on the upstream side of the filter on a basis of both results of determination by the first and second, frequency determination means, wherein the contamination level determination means is for correcting effect of miss-sensing on the first light-blocking sensor by comparing a result of determination of the sensing frequency of foreign matter of particle sizes by the first frequency determination means with a result of determination of the sensing frequency of foreign matter of the same particle sizes by the second frequency determination means. a pilot filter arranged in a pilot line of the hydraulic circuit to remove, from the hydraulic oil, foreign matter that may cause an operation failure of the hydraulic working machine, a first light-blocking sensor for sensing passage of foreign matter on an upstream side of the filter, a second light-blocking sensor for sensing passage of foreign matter on a downstream side of the filter, wherein the onboard controller unit is designed to function as: a first frequency determination means for determining a frequency of sensing of foreign matter by the first light-blocking sensor, a second frequency determination means for determining a frequency of sensing of foreign matter by the second light-blocking sensor, a contamination level determination means for determining a contamination level on the upstream side of the filter on a basis of both results of determination by the first and second, frequency determination means, wherein the contamination level determination means is for correcting effect of miss-sensing on the first light-blocking sensor by comparing a result of determination of the sensing frequency of foreign matter of particle sizes by the first frequency determination means with a result of determination of the sensing frequency of foreign matter of the same particle sizes by the second frequency determination means. a first frequency determination means for determining a frequency of sensing of foreign matter by the first light-blocking sensor, a second frequency determination means for determining a frequency of sensing of foreign matter by the second light-blocking sensor, a contamination level determination means for determining a contamination level on the upstream side of the filter on a basis of both results of determination by the first and second, frequency determination means, wherein the contamination level determination means is for correcting effect of miss-sensing on the first light-blocking sensor by comparing a result of determination of the sensing frequency of foreign matter of particle sizes by the first frequency determination means with a result of determination of the sensing frequency of foreign matter of the same particle sizes by the second frequency determination means.