A blow-by gas recirculation device configured to guide blow-by gas from a crankcase to an intake passage through an in-cover gas passage formed inside a head cover, wherein an oil separator that traps and removes oil from the blow-by gas is attached to an inside of the head cover, a pressure regulating valve is provided on an outlet side of the in-cover gas passage in the head cover, and a separator outlet, which is an outlet for the blow-by gas in the oil separator, is overlapped on a blow-by gas inlet portion of the pressure regulating valve, wherein the oil separator includes a separator inlet through which the blow-by gas is introduced, an oil filter, an oil dropping portion, and the separator outlet through which the blow-by gas is discharged, the oil separator includes a separator rear portion having the separator inlet, a separator middle portion, the oil filter located at a boundary between the separator rear portion and the separator middle portion, a separator front portion having the separator outlet, and a partition wall, when a longitudinal direction of the head cover is defined as a front-rear direction, with one of the front-rear direction being front and another one being rear, characterized in that: the separator front portion is provided with the separator outlet in an upper portion and a downward protrusion which serves as the oil dropping portion in a lower portion, the partition wall extends in the front-rear direction in both the separator front portion and the separator middle portion and is positioned between the separator outlet and the oil dropping portion, the partition wall is provided with a droplet collecting portion and a bottom of the droplet collecting portion is provided with a through hole in the separator middle portion, and oil collected in the droplet collecting portion returns into the engine via the through hole and the oil dropping portion. an oil separator that traps and removes oil from the blow-by gas is attached to an inside of the head cover, a pressure regulating valve is provided on an outlet side of the in-cover gas passage in the head cover, and a separator outlet, which is an outlet for the blow-by gas in the oil separator, is overlapped on a blow-by gas inlet portion of the pressure regulating valve, wherein the oil separator includes a separator inlet through which the blow-by gas is introduced, an oil filter, an oil dropping portion, and the separator outlet through which the blow-by gas is discharged, the oil separator includes a separator rear portion having the separator inlet, a separator middle portion, the oil filter located at a boundary between the separator rear portion and the separator middle portion, a separator front portion having the separator outlet, and a partition wall, when a longitudinal direction of the head cover is defined as a front-rear direction, with one of the front-rear direction being front and another one being rear, characterized in that: the separator front portion is provided with the separator outlet in an upper portion and a downward protrusion which serves as the oil dropping portion in a lower portion, the partition wall extends in the front-rear direction in both the separator front portion and the separator middle portion and is positioned between the separator outlet and the oil dropping portion, the partition wall is provided with a droplet collecting portion and a bottom of the droplet collecting portion is provided with a through hole in the separator middle portion, and oil collected in the droplet collecting portion returns into the engine via the through hole and the oil dropping portion.