To enhance the noise reducing effect of a sound insulation structure which includes a cover member for covering a power unit from the outside so as to reduce the radiant sound generated from the power unit, and a resonator. A sound insulation structure includes an armor cover for covering a left case half and a left cover constituting a side part cover of the power unit from the left side so as to reduce the radiant sound generated from the side part cover and enhance an appearance quality of the power unit, and the resonator capable of resonating at a specified frequency of the radiant sound. The resonator is disposed in a space formed between the side part cover and the armor cover in the state of being out of contact with the armor cover, and is attached to the side part cover in contact with the latter.