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 (B) 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 (B), 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 (B) 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 (B), 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 . <EOS>