An oil filtering device (1) for use in a swing-type power unit (8) of a motor bicycle (2) having a four-stroke-cycle engine (4) including a crank case (26), comprises an oil pan (12) for storing oil (11), an oil pump (18) for conveying the oil (11) in the oil pan (12) to engine portions such as a crank shaft (16) involving friction, a lubrication passage through which the oil (11) conveyed by the oil pump (18) is supplied to the engine portions involving friction, an oil strainer (22) provided in the lubrication passage (20) for first filtering the oil (11), and an oil filter (24) for further filtering the oil (11) which has been filtered by the oil strainer (22). The oil (11) to be fed to the crank shaft (16), for example, involving friction is conveyed by way of the oil strainer (22) and the oil filter (24) through the lubrication passage (20) before being supplied thereto. The oil strainer (22) and the oil filter (24) are arranged below and spaced apart from the crank shaft (16) and detachably provided on a surface of the crank case (26) composed of two halves. The swing-type power unit (8) is provided with a suspension arm (8a) having a suspension boss (29) to which the oil filter (24) is provided.