A suction air volume detecting means to be provided in a suction system in an engine, in particular an automotive engine to electrically detect the air volume to be sucked into the engine is mounted within a suction air expansion chamber of the suction system provided to reduce the pulsation of the suctioned air for the purpose that the electrical circuit elements of the suction air volume detecting means, in particular, transistors, etc. are protected from being heated by heat radiated from the engine and from being caused to misoperate by external electrical waves.