PURPOSE: To manufacture a device particularly suitable for a constraction vehicle by constituting integrally a removable cassette in which two or more filters are inserted, a specified actuation means and a passage bypassing hydraulic oil.

CONSTITUTION: A plate 2 is inserted into a container 1, and filters 1-1, 1-2 and 1-3 of different filter grain diameter are clamped between said container and plate to form one cassette A. Said cassette A is pushed upward by a spring 23, and an oil hole of a cap 5 is brought into contact with a tapered section 22 of a cover 20 to seal the oil. In case a filter is replaced, the cover 20 is removed from a case 10 to remove the cassette A by means of the spring 23. The replacement of filter is carried out by every cassette A and can be carried out in a short period of time. When the filter is clogged to raise the pressure and pressure difference between an inlet of the first filter and an outlet of the last filter reaches the set value, the push-up force of the spring 23 is reduced to open a contact section of the tapered section 22 and the cap 5 and the oil flows from a chamber 25 to a piping 30 through passages 24 and 26 to carry out the function of a bypass.