A filter comprising a composite sheet bent like a pleat and composed of a laminate of at least two monolayer sheets, in which the bent portions are fusion-welded, is provided in order to improve filtering properties while carrying out the composing and pleating processes in one process. A device for manufacturing the filter is also provided. The device comprises, for example: a pair of star-shaped gear type ribbed rolls engaged with each other for heating the composite sheet to temperatures suitable to fusion-weld the same at the front end of the gear while bending the composite sheet like a pleat having a bent width corresponding to the depth or height of the gear; star-shaped gear type bending roll for further bending the heated composite sheet like a pleat; and a pair of heating plates positioned opposed each other at a distance corresponding to the bent width of the pleat. In this device, the composite sheet is selectively and linearly heated and fusion-welded at the bent portions in contact with the front end of the gear while being bent, and it is further bent by the bending roll and then passed through between the heating plates where the composite sheet is heated and fusion-welded at the bent portions from the outer side thereof.