An indwelling catheter has a hollow catheter body, a catheter tube fitted to a front end of the catheter body, an elastic valve fitted inside the catheter body, and a hollow plug slidably fitted inside the catheter body. The diameter of the nut cap is larger than the diameter of the assembling portion. The electronic system monitors the buttons selected by a user to determine the state of all external electronic devices that are to be controlled. Thus, since a voltage V3 generated at the resistor R3 becomes a voltage from which the influence of the offset voltage Voff is eliminated, it is possible to determine an overcurrent accurately by eliminating the influence of the offset voltage Voff.