A working machine monitoring system including a plurality of working machines and a monitoring server connected to the plurality of working machines through a network, the working machine monitoring system comprising: - an engine operation parameter acquisition module that acquires an engine operation parameter representing the operation state of an engine mounted on each of the working machines; - a refueling time acquisition module that acquires a refueling time when each of the working machines is supplied with fuel; - a fuel property determination module that determines the engine operation parameter changes in dependence on change of the property of the fuel when a lapsed time from refueling of fuel is within a first time period in which an influence by the fuel property is considered to arise and when a change amount of the engine operation parameter exceeds a first threshold value, based on a result of comparison of a time when the engine operation parameter has deviated from a reference value corresponding to prescribed fuel, with the refueling time; - terminal-side communication control modules respectively provided on the plurality of working machines for transmitting to the monitoring server individual information being information as to a property determination processing for the fuel in each of the working machines and a server-side communication control module that receives the individual information transmitted from the terminal-side communication control modules; and - a fuel property final determination module that chooses a plurality of individual information satisfying a condition that enables a same fuel to be considered as having been refueled, from individual information including the fuel property determination results received from the respective working machines and engine operation parameters, and executes a final determination of the fuel property by the execution of a majority rule processing or the like of the individual information, wherein: based on a result of the final determination, the server-side communication control module transmits an instruction information for instructing a restriction in output power of the engine for the working machine being an object to be determined; and the terminal-side communication control module provided on the working machine being the object to be determined receives the instruction information. - an engine operation parameter acquisition module that acquires an engine operation parameter representing the operation state of an engine mounted on each of the working machines; - a refueling time acquisition module that acquires a refueling time when each of the working machines is supplied with fuel; - a fuel property determination module that determines the engine operation parameter changes in dependence on change of the property of the fuel when a lapsed time from refueling of fuel is within a first time period in which an influence by the fuel property is considered to arise and when a change amount of the engine operation parameter exceeds a first threshold value, based on a result of comparison of a time when the engine operation parameter has deviated from a reference value corresponding to prescribed fuel, with the refueling time; - terminal-side communication control modules respectively provided on the plurality of working machines for transmitting to the monitoring server individual information being information as to a property determination processing for the fuel in each of the working machines and a server-side communication control module that receives the individual information transmitted from the terminal-side communication control modules; and - a fuel property final determination module that chooses a plurality of individual information satisfying a condition that enables a same fuel to be considered as having been refueled, from individual information including the fuel property determination results received from the respective working machines and engine operation parameters, and executes a final determination of the fuel property by the execution of a majority rule processing or the like of the individual information, wherein: based on a result of the final determination, the server-side communication control module transmits an instruction information for instructing a restriction in output power of the engine for the working machine being an object to be determined; and the terminal-side communication control module provided on the working machine being the object to be determined receives the instruction information.