To provide an oil separator including multiple cones and having a simple structure and high trapping efficiency for oil mist. An oil separator can include multiple cones arranged in a line and a chamber for blow-by gas, which is formed in adjacent to the line of the cones, and arranged at an upper side surface of the cones, the chamber being provided with a flow-in port at a position in front of the line of the cones, in which upper outer peripheral walls of the cones at portions facing the chamber constitute partition walls provided with slits, which constitute inlets for the blow-by gas to enter the cones.