An inlet air filter device (1) comprises: at least a passage (3) for delivering an air mass (4) from an inlet section (5) to an outlet section (6) of the inlet air filter device (1), a water separator (10) in the passage (3) for separating water in the air mass (4), the water separator (10) being positioned between the inlet section (5) and the outlet section (6), a bypass element (15, 115) movable between a first open position in which the air mass (4) is forced to cross the water separator (10) and a second closed position in which the air mass (4) flows from the inlet (5) to the outlet section (6) without crossing the water separator (10), a shutter element (13) which is actuatable to co-operate with the bypass element (15, 115) for isolating the water separator (10) from the air mass (4) when the bypass element (15, 115) is in the second closed position.