Abstract: An engine supercharging device includes are a supercharger (38) for pressurizing air introduced in an engine (E), an air cleaner (36) for purifying an ambient air, a purified air supply passage (56) for supplying a purified air (CA) from the air cleaner (36) towards the supercharger (38), a supercharged air passage (62) for supplying the supercharged air (SA) from the supercharger (38) towards an air intake passage (60) of the engine (E), and a relief valve (68) for adjusting an air pressure within the supercharged air passage (62). The relief valve (68) has a discharge port portion (68b) which is accommodated within the air cleaner (36). The engine (E) has a plurality of engine cylinders, and a downstream portion of the supercharged air passage (62) is defined by an intake air chamber (54) for supplying the sucked air to respective air intake passages (60) of the plural engine cylinders.