An apparatus is provided for detecting a malfunction in an on-vehicle charging system provided with a generator driven by an engine and a battery connected with the generator via a charging line. The apparatus comprises a first control unit controlling a generated condition of the generator and including a first voltage detector detecting a voltage generated by the generator and a second control unit including a second voltage detector detecting a voltage across the battery. The apparatus further comprises a communication device communicating, in the form of digital signals, via a communication line between the first and second control units, information about both the generated voltage and the battery voltage and a malfunction determination unit determining whether or not a malfunction occurs in the charging system on the basis of the communicated information.