A water quality analyzer comprises: sensor electrodes 1a, 1b made of different metals from each other, the electrodes in a liquid of inspecting object generating a sense voltage in proportion to the liquid's impurities concentration; an operational amplifier OP1 amplifying the sense voltage without inverting to provide for a CPU 3; a resistor R0 whose one end is connected to the electrode 1a; and a voltage divider 2 applying a voltage obtained by dividing the sense voltage by a prescribed division ratio to R0's another end. The CPU 3 calculates input signal from OP1 to obtain chlorine concentration and displays the calculated result on a LCD 4 in a measurement mode, and sets the division ratio of the divider 2 so that sense voltage across electrodes 1a, 1b soaked in a liquid including prescribed concentration chloride approximately agrees with a reference voltage of prescribed concentration in a sense-voltage calibration mode.