An oil separator for a blow-by gas containing oil, comprising: a gas flow passage, provided with a gas inlet port and a gas outlet port for the blow-by gas, for allowing a flow of the blow-by gas from the gas inlet port towards the gas outlet port; separation means (44, 46), provided midway along the gas flow passage, for separating the oil from within the blow-by gas which flows inside the gas flow passage; an oil passage, at least a part of which is formed as a common flow path section located downstream of the separation means in the blow-by gas flowing direction and shared by the gas flow passage, for allowing the oil separated by the separation means to flow along a bottom of the common flow path section; and an oil discharge port, provided at a downstream side section, in the oil flowing direction, of the oil passage, for discharging the oil which flows inside the oil passage to the outside of the oil passage, wherein a flow passage regulating means is provided in the common flow path section so as to extend upwards from the bottom of the common flow path section, and wherein the flow passage regulating means makes a part of the common flow path section a restricted portion, by regulating a width of the part of the common flow path section so that the width becomes gradually narrower towards the bottom of the common flow path section, characterized in that the flow passage regulating means is formed by a single plate-shaped rib, substantially half section of one side in the width direction thereof having a substantially trapezoidal shape with a height gradually reducing going towards another side in the width direction along a straight, curved or stepped sloping section, while the remaining substantially half section has a rectangular shape extending at the same height, wherein the single plate-shaped rib is shorter than the length of the common flow path. a gas flow passage, provided with a gas inlet port and a gas outlet port for the blow-by gas, for allowing a flow of the blow-by gas from the gas inlet port towards the gas outlet port; separation means (44, 46), provided midway along the gas flow passage, for separating the oil from within the blow-by gas which flows inside the gas flow passage; an oil passage, at least a part of which is formed as a common flow path section located downstream of the separation means in the blow-by gas flowing direction and shared by the gas flow passage, for allowing the oil separated by the separation means to flow along a bottom of the common flow path section; and an oil discharge port, provided at a downstream side section, in the oil flowing direction, of the oil passage, for discharging the oil which flows inside the oil passage to the outside of the oil passage, wherein a flow passage regulating means is provided in the common flow path section so as to extend upwards from the bottom of the common flow path section, and wherein the flow passage regulating means makes a part of the common flow path section a restricted portion, by regulating a width of the part of the common flow path section so that the width becomes gradually narrower towards the bottom of the common flow path section, characterized in that the flow passage regulating means is formed by a single plate-shaped rib, substantially half section of one side in the width direction thereof having a substantially trapezoidal shape with a height gradually reducing going towards another side in the width direction along a straight, curved or stepped sloping section, while the remaining substantially half section has a rectangular shape extending at the same height, wherein the single plate-shaped rib is shorter than the length of the common flow path.