To collect dust contained in oil even in high viscosity of the oil and to prevent damage of a filter element, without increasing a space for attaching, the number of part items, and assembling manday.

A cylindrical filter element 16 provided on an element plate 17 is stored in a filter case 14, and the filter case 14 is attached to an underside of a filter housing 12. A thorough hole 17b is formed facing to the inside of the filter element 16 in the element plate 17, and a safety valve 28 for opening and closing the hole 17a in response to oil pressure in the outside of the filter element 16 is stored inside the filter element 16. A filter net 29 is fitted to the element plate 17 or the safety valve 28 to cover the safety valve 28.