As a further improvement of the technique of minimizing or preventing excessive cooling of an oil separator, an effective blow-by gas heating apparatus which minimizes or solves the problems due to frozen blow-by gas in an engine externally equipped with an oil separator is provided. The blow-by gas heating apparatus includes: a heat emitting structure abutted onto an oil separator configured to trap and remove oil from blow-by gas. The heat emitting structure has a heat emitting case including inside a passage for engine cooling water. The heat emitting case includes a ceiling wall being in surface-contact from below with a bottom surface of the oil separator.