A natural gas engine capable of exhaust emission control over the entire temperature range and at low costs without requiring a complicated structure. An adsorbent (7) is disposed between a fuel tank (4) and a fuel supply port (5) on a suction air passage (2), and NMOG in the fuel gas supplied to the fuel tank (4) is adsorbed by the adsorbent (7) to purify the fuel gas in advance. When the temperature of a catalyst (11) disposed on an exhaust passage (3) reaches an activation temperature, the adsorbent (7) is heated by a heater (9) to a desorption temperature to desorb NMOG from the adsorbent (7) and purify the NMOG by the catalyst (11).