To make a filter high filtration performance at low cost by alternately continuing angled bent parts and flat parts made of a nonwoven fabric, making the density of the bent parts smaller than that of the flat parts following them, making the density of the flat parts smaller than that of the following bent parts and forming the density up to the final bent parts and flat parts successively larger.

A bent part 1 of the smallest density D1 is spread in the outermost part on the gas inflow side of a frame body 3, a flat part 2 following the bent part 1 is spread on the gas outflow side of the bent part 1, a bent part 11 and a flat part 21 following it are spread on the gas outflow side of the flat part 2 and the bent parts and the flat parts whose the densities are successively made larger after those are alternately spread in the frame body 3 leaving a space. The density of the bent part 1 spread in the outermost part on the gas inflow side and that of a flat part 2n spread in the outermost part on the gas outflow side are determined by the size of fine particles caught from the fluid to be treated. The use of a separator is not needed, fitting work is simple and fine particles from large diameter down to small diameter in the fluid can be caught in each stage of the different density to provide high filtration performance.