PURPOSE: To obtain the reducing device for the in-cabin noise which can generate all higher harmonic components by generating higher harmonic signals of engine rotation half order.

CONSTITUTION: The reference signal from a means 12 which detects the reference signal synchronized with the vibration of an engine 11 is not inputted directly to a controller 2, but inputted to a higher harmonic generating device 3 temporarily, which generates various higher harmonic signals of half cycles of the engine rotating speed, i.e., (0.5)th order. Further, the higher harmonic generating device 3 selects and adds desired higher harmonic signals among the higher harmonic signal of (0.5)th order of the engine rotation and supplies the result to the controller 2. In this case, noise reduction for the tone quality in various cabins 10 can be performed by previously finding desired higher harmonic signals by measurements.