A battery checker utilizes a reference voltage generator device which detects a voltage corresponding to an energy gap of a semiconductor, or a voltage of a value close thereto, or a voltage based on an energy level of a semiconductor, and generates the detected voltage as a reference voltage. The reference voltage is generated by detecting a difference of threshold voltages of first and second insulated gate field-effect transistors (IGFETs). Gate electrodes of the first and second IGFETs are formed on gate insulating films which are formed on different surface areas of an identical semiconductor substrate under substantially the same conditions. The gate electrodes of the first and second IGFETs are respectively made of two semiconductors which are selected from among a semiconductor of a first conductivity type, a semiconductor of a second conductivity type and an intrinsic semiconductor made of an identical semiconductor material, but which have Fermi energy levels of values different from each other.