In an engine unit sensor installation structure, a partition wall (63) that separates a crank chamber (17a) from an inner space of an engine cover (51) is provided on an opening (17b) of a crankcase (17). A rotor (52) is attached to an end portion of a crankshaft (16) such that a bottom wall (52a) is located on the crankcase (17) side. A stator (53) of an ACG starter (50) and a Hall IC unit (57) of a position detection sensor (58) are attached to the engine cover (51). A fin (67) for forced cooling used to introduce a cooling air to a space between a partition wall (63) and a bottom wall (52a) of the rotor (52) is provided between the partition wall (63) and the rotor (52).