To ensure satisfactory sound absorbing effect, regardless of the frequency of a sound to be absorbed.

An internal combustion engine mainbody 1 is encircled by a sound insulating device 2. The sound insulating device 2 has a piezoelectric element 3, a pair of electrodes 4 and a variable resistor 5 connected between the electrodes 4. Sound energy is converted into electric energy by the piezoelectric element 3 and the electric energy is consumed as thermal energy in the variable resistor 5, thereby insulating a sound. The frequency of a sound to be effectively absorbed by the sound insulating device 2 is determined by an electric resistance value for the variable resistor 5. So, the rotating number of an engine is detected, the frequency of a sound to be absorbed from the internal combustion engine is specified in accordance with the rotating number of the engine and the electric resistance value for the variable resistor 5 is controlled for the sound insulating device 3 to provide the frequency of a sound to be most effectively absorbed.