disclosed is an integrated ion sensor , which comprises at least one ion selective membrane sensitive to ions contained in a solution to be measured for detecting the concentration of the ions , a signal processing circuit for inputting a detected signal obtained by the ion selective membrane through a conductive member and fetching the detected signal through mosfets or the like included in an input stage to process the same , a reference electrode disposed in the measuring environment made by the solution to be measured and to be set to a predetermined voltage relationship between the reference electrode and the ion selective membrane , and a power supply having negative and positive terminals for supplying a driving power to the signal processing circuit through the terminals , one of the terminals being connected to the reference electrode , wherein the signal processing circuit is set to an active state at a voltage set to the reference electrode by controlling the threshold value of at least one of the mosfets of the signal processing circuit .