An internal combustion engine is provided which can operate by either the homogeneous charge compression ignition system or the spark ignition system using a single fuel. In the internal combustion engine using a fuel containing a hydrocarbon, the fuel contains o-xylene of 10 to 70 wt. % of the total weight, and the engine operates by either homogeneous charge compression ignition or spark ignition depending on the load. The fuel contains o-xylene of 30 to 50 wt. % of the total weight. The fuel contains, for example, n-heptane as the base fuel, and o-xylene of 10 to 70 wt. %, preferably 30 to 50 wt. %, of the total base fuel. The internal combustion engine operates by homogeneous charge compression ignition at low or medium load and by spark ignition at high load.