Internal combustion engine

An internal combustion engine comprising:

• a crankcase that defines a crank chamber;
• a crankshaft that has a journal linked to a bearing and is rotatably supported on the crankcase;
• a power transmission gear fixed to an extremity of the crankshaft that has a smaller diameter than a diameter of the journal and projects outward of the crankcase;
• a to-be-detected body that is provided on an outer periphery of a collar member sandwiched between the bearing and the power transmission gear and is relatively non-rotatably supported on the crankshaft;
• a detection sensor that is made to face a trajectory of the to-be-detected body and detects movement of the to-be-detected body to generate a pulse signal;
• a key groove formed in inner peripheral faces of the power transmission gear and the collar member and extending in an axial direction of the crankshaft;
• and
• a key member projecting from an outer periphery of the crankshaft and fitted into the key groove,
• wherein the key member engages with the key groove and is disposed between both the power transmission gear and collar member and the crankshaft.