PURPOSE: To measure the H-infinite norme of a controlled system in real time and also to automatically monitor the safety of the controlled system.

CONSTITUTION: A safety monitor device 1 of a control system is provided with model response arithmetic means 4 which sequentially sequentially operates the estimated value of a controlled variable from the estimated value of a manipulated variable based on the linear model of a controlled system 70, a arithmetic means 5 which sequentially operates the signal series of the model difference, i.e., the difference between the estimated value of the controlled variable and measurement value. a filter means 6 which has a transmission function, T=(I+-CP)-1C where P shows the transmission function of the linear model of the system 70 with C meaning the transmission function of a controller 60 and I showing a unit matrix respectively and then uses the signal series of the model difference as an input signal, a measurement device 10 which measures H-indefinite norme of a system where the signal series of the model difference is used as an input signal and the input of the means 6 used as an output signal, a decider means 30 which decides that a feedback control system is set in a robust stable state and in a robust unstable state when the H-infinite norme is smaller and larger than 1 respectively.