Balance shaft, housing for balance shaft and engine oil return passage

An engine including an engine oil return passage for returning engine oil from a cylinder head side to an oil pan (5) has: a balance shaft (13L, 13R) forming a balancing system (4) for said engine; and a housing (14U, 14L) disposed below a crankshaft (1) for accommodating rotatably therein said balance shaft (13L, 13R). A part (142) of said engine oil return passage is provided in said housing (14U, 14L)