A monitor system comprising:an adder for summing up a plurality of unknown analog electric quantity signals denoting the operating conditions of such as a transmission line;a multiplexer for generating output signals corresponding to the plural analog electric quantity signals and also to a signal denoting the result of addition made by said adder;an A-D converter for converting output analog signals from the multiplexer into digital signals; anda monitor which sums up those of the output digital signals from the A-D converter which correspond to said plural analog electric quantity signals, compares a signal denoting the result of said addition with a digital signal converted from an output signal from the adder, receives an error detection signal denoting the known prescribed electric quantity converted into a digital signal through the multiplexer and A-D converter, and judges the operating condition of the A-D converter from the contents of said received digital signal.