An apparatus includes a lubricating oil tank storing a lubricating oil. An oil separator separates the lubricating oil from a working fluid discharged from an expander of a waste heat recovery system. A lubricating oil return conduit is connected between the oil separator and the lubricating oil tank. A working fluid return conduit is connected between the lubricating oil tank and the oil separator. A vaporizer unit is installed in the lubricating oil tank to vaporize a liquefied working fluid mixed with the separated lubricating oil in the lubricating oil tank.