the laminated non - woven fabric for use in a filter of the present invention may be produced by stacking a plurality of non - woven fabric layers having different average pore sizes in the order of the average pore size , and spraying the water jet onto one of the surfaces of the resulting stack . in the following , a method for producing a laminated non - woven fabric of two layers ( first non - woven fabric and second non - woven fabric ) will be described . however , it is to be understood that the present invention is not intended to be limited to the two - layered laminated non - woven fabric . the thermoplastic resin for fibers constituting the non - woven fabric used in the present invention is not critically restricted , and may include polyolefin such as polyethylene , polypropylene , etc ., polyester such as polyethylene terephthalate , polybutylene terephthalate , etc ., polyamide such as nylon 6 , nylon 66 , nylon 46 , etc ., and other polymers such as polyvinyl chloride , polyvinylidene chloride , polystyrene , polycarbonate , polyvinylidene fluoride , etc . in particular , polyolefin and polyamide are preferable . the non - woven fabric to be used in the present invention is not practically restricted as long as made of the thermoplastic resin exemplified above , and may be produced by a known method such as melt blow method , spun bonding method , etc . a melt - blown non - woven fabric is particularly preferable in view of filtering efficiency of a resulting filter . the first non - woven fabric is preferred to have an average pore size of 5 - 30 μm and a thickness of 0 . 3 - 2 . 0 mm , and the second non - woven fabric to have an average pore size of 20 - 100 μm and a thickness of 1 - 8 mm . the average pore size of the second non - woven fabric is preferred to be larger than that of the first non - woven fabric by 50 - 70 μm . a basis weight , gas permeability and average fiber diameter of each of the first and second non - woven fabrics are preferably 20 - 100 g / m 2 , 15 - 300 cc / cm 2 / sec and 0 . 5 - 20 μm , respectively . the average pore size depends on the average fiber diameter and / or the packing density of the non - woven fabric . generally , the larger the average fiber diameter , the larger the average pore size . on the other hand , the larger the packing density , the smaller the average pore size . in the present invention , it is preferred to use a plurality of non - woven fabric layers which are different from each other in both the average pore size and the average fiber diameter . the first and second non - woven fabrics are stacked each other , and one of the surface of the resulting stack is sprayed with the water jet . the water jet process is illustrated in fig1 in which the water jet apparatus comprises a water jet rod 1 having a plurality of axially spaced nozzles , which is disposed above the stack of the first non - woven fabric 2 and the second non - woven fabric 3 so as to transversely across the stack while facing the nozzles to the stack . either of the first non - woven fabric 2 or the second non - woven fabric 3 may be uppermost , however , it is generally p referable to place the stack below the water jet rod 1 so that the first non - woven fabric 2 is upstream and the non - woven fabric 3 is downstream with respect to the flow path of the water jet 11 because fibers of a smaller average fiber diameter more easily penetrate into the non - woven fabric layer of a larger average fiber diameter . the number of the non - woven fabric layers to be stacked is not limited to two . a stack of three layers , four layers or more layers may be sprayed with the water jet to produce a laminated non - woven fabric of three , four or more layers . in case of producing a laminated non - woven fabric of three or more layers , the average fiber diameter , basis weight and gas permeability of each of the third non - woven fabric and the following non - woven fabric layers may be independently selected from the ranges mentioned above as far as the non - woven fabric layers are stacked in the order of average pore size . the average pore size and the thickness of such third and the following non - woven fabric layers are preferably 30 - 120 μm and 1 - 10 mm , respectively . the spray pressure of the water jet 11 from the water jet rod 1 is generally 20 - 200 kg / cm 2 , preferably 50 - 150 kg / cm 2 depending upon the kinds of non - woven fabric and / or the construction of the stack . a spray pressure less than 20 kg / cm 2 results in insufficient bonding of the non - woven fabric layers . on the other hand , when the spray pressure exceeds 200 kg / cm 2 , the non - woven fabric comes to be likely broken during spraying the water jet 11 . the inter - nozzle spacing is preferably 0 . 3 - 2 mm , more preferably 0 . 3 - 1 mm . an inter - nozzle spacing less than 0 . 3 mm makes the nozzle plate less pressure resistant . when the inter - nozzle spacing exceeds 2 mm , the bonding between the non - woven fabric layers becomes insufficient because of small bonding area ( small number of entangled regions ) and the surface of the sprayed non - woven fabric is roughened . the spray amount of the water jet 11 is 500 - 1500 liter / min , preferably 600 - 1200 liter / min per 1 m width . the stack of the non - woven fabric layers 2 and 3 moves in the direction indicated by an arrow a shown in fig1 while spraying the water jet 11 onto the upper surface of the stack . the feeding speed of the stack is preferably 5 - 80 m / min , more preferably 10 - 50 m / min . the productivity of the laminated non - woven fabric is reduced when the feeding speed is less than 5 m / min . on the other hand , a feeding speed exceeding 80 m / min results in insufficient bonding between the non - woven fabric layers . the water jet rod 1 may be reciprocated in the direction nearly perpendicular to the feeding direction ( the direction indicated by an arrow b of fig1 ) of the stack so as to increase the bonding area . the spraying of the water jet may be also conducted by scanning a water jet rod having only one nozzle longitudinally and transversely with respect to the stack in stead of using the water jet rod 1 described above . the bonding strength between each of the adjacent layers of the laminated non - woven fabric produced as described above is 0 . 5 kgf / 50 mm width , preferably 1 - 7 kgf / 50 mm width when measured by a 180 ° peeling test with a tensile strength tester . a schematic cross sectional view of one of the laminated non - woven fabric according to the present invention , which is taken along the transverse direction of the laminated non - woven fabric , is shown in fig2 . in fig2 the laminated non - woven fabric 20 has three layers . a first layer 21 consists of the first non - woven fabric 2 , and a second layer 22 consists of the second non - woven fabric 3 . between the first layer 21 and the second layer 22 , a third layer 23 is interposed . the third layer 23 consists of the entangled region 23a which comprises the fiber constituting the first non - woven fabric ( the first fiber ) penetrated into the second non - woven fabric by pressure of the water jet 11 and entangled with the fiber constituting the second non - woven fabric ( the second fiber ), and the non - entangled region 23b which comprises only the second fiber . a stream of the water jet 11 from a nozzle reaching the surface of the first layer 21 forms there a recess 21a by its pressure , and then passes through the first layer 21 along the direction shown by an arrow c to penetrate the first fiber into the second non - woven fabric . the penetrated first fiber is entangled with the second fiber to form the entangled region 23a . thus , a pair of the recess 21a and the entangled region 23a is formed along the flow path of the water jet 11 . the first and second layers 21 and 22 are substantially identical with the first and second non - woven fabrics 2 and 3 in their characteristics , and therefore , the average pore size of each of the layers 21 and 22 is the same as that of each of the non - woven fabrics 2 and 3 . the average pore size of the third layer 23 is preferably 5 - 100 μm , more preferably 10 - 40 μm . the area ratio of the entangled region 23a ( bonding area ratio ) is 5 % or more , preferably 50 % or more , and the practically available upper limit is usually about 80 %. the area ratio is calculated based on a cross section of the laminated non - woven fabric taken along the transverse direction thereof , and expressed by the ratio of the total area of the entangled region 23a on the cross section to the total area of the cross section . an area ratio less than 5 % is disadvantageous because sufficient bonding strength between the non - woven fabric layers cannot be attained as well as the pressure loss is arisen at the non - entangled region 23b which causes the peeling of the laminated non - woven fabric layers and accumulation of solid matters . in the third layer 23 , the portion nearest the boundary between the first and third layers 21 and 23 is rich in the first fiber . the content of the first fiber decreases downwardly , and the third layer 23 changes to the second layer 22 at the portion where the first fiber comes to be not present . in the laminated non - woven fabric 20 as shown in fig2 the thickness of the first layer 21 is 0 . 1 - 2 . 0 mm , preferably 0 . 1 - 1 . 0 mm , and 0 . 1 - 1 . 0 mm , preferably 0 . 5 - 1 . 0 mm for the second layer 22 . the third layer 23 is preferred to have a thickness of 1 - 8 mm , more preferably 1 - 5 mm . the thickness of the third layer 23 is preferably 5 - 80 % , more preferably 50 - 80 % of the total thickness of the laminated non - woven fabric . the total thickness of the laminated non - woven fabric is preferably 0 . 5 - 3 . 0 mm , more preferably 1 . 0 - 2 . 5 mm . the laminated non - woven fabric 20 of the present invention is used as a filtering medium while preferably setting the second layer 22 having a larger average pore size as the inlet side and the first layer 21 as the outlet side . the laminated non - woven fabric has a fibrous multi - layer construction in which the average pore size gradually changes from one surface to the other surface and has no portion where the average pore size changes abruptly . therefore , a filter made of the laminated non - woven fabric of the present invention has a good filtering efficiency and suffers from no accumulation of solid matters at the bonded interface , thereby elongating the lifetime of the filter . the non - woven fabric of the present invention is preferred to have the following additional filtering characteristics : ( 2 ) life : 500 - 3000 cc · 2 kgf / cm 2 or more , more preferably 1000 - 3000 cc · 2 kgf / cm 2 . the initial efficiency is represented by the ratio of the decrease in turbidity by passing through a filter made of the laminated non - woven fabric to the initial value of turbidity of a liquid . the life is represented by the total volume of a liquid passing through a unit surface area ( cm 2 ) of the filter until the pressure loss reaches 2 kgf . therefore , the larger the value of the life , the lower the pressure loss . the present invention will be further described while referring to the following examples which should be considered to illustrate various preferred embodiments of the present invention . in the following examples and comparative examples , the following melt - blown non - woven fabrics made of polypropylene were used . ______________________________________basis thick - gas average averageweight ness permeability pore size fiber dia -( g / m . sup . 2 ) ( mm ) ( cc / cm . sup . 2 / sec ) ( μm ) meter ( μm ) ______________________________________ppnw - 1 30 0 . 56 17 14 0 . 8ppnw - 2 30 0 . 56 34 20 1 . 5ppnw - 3 40 0 . 64 50 30 5ppnw - 4 100 1 . 2 42 40 8______________________________________ each of the properties of the non - woven fabric was measured or calculated as follows . ( 2 ) gas permeability : measured according to jis l1096 using a frasil test machine . ( 4 ) initial efficiency : the turbidity ( τ ) of a fluid before and after filtering w as calculated from the following equation : wherein i 0 is the intensity of incident light , i is the intensity of transmitted light , and l is the optical path . the initial efficiency was calculated from the following equation : wherein τ in and τ out are turbidities before and after filtering , respectively . each stack of three or four melt - blown non - woven fabrics ( ppnw - 1 to ppnw - 4 ), as shown in table 1 , was boned each other by water jet under the conditions shown in table 1 to produce each laminated non - woven fabric . the thickness , average pore size , initial efficiency , and life of each laminated non - woven fabric are shown in table 1 . table 1______________________________________ example example example example 1 2 3 4______________________________________production conditionsnon - woven fabricfirst layer ppnw - 1 ppnw - 1 ppnw - 2 ppnw - 2second layer ppnw - 2 ppnw - 3 ppnw - 3 ppnw - 3third layer ppnw - 3 ppnw - 4 ppnw - 4 -- fourth layer ppnw - 4 -- -- -- spray pressure of 80 80 80 80water jet ( kg / cm . sup . 2 ) spray amount of 800 800 800 800water jet ( liter / min / 1m width ) nozzle spacing ( cm ) 0 . 3 0 . 3 0 . 3 0 . 3feeding speed ( m / 20 20 20 20min ) bonding area ratio 50 50 50 50 (%) propertiesthickness ( mm ) 1 . 88 1 . 60 1 . 89 0 . 73gas permeability 4 . 0 6 . 0 7 . 1 11 . 8 ( cc / cm . sup . 2 / sec ) average pore size 15 18 18 19 ( μm ) initial efficiency (%) 74 53 44 35life ( cc · 2 kgf / cm . sup . 2 ) 900 915 1450 1430______________________________________ the same procedures as in examples 1 - 4 were repeated except that each of the non - woven fabric layers were bonded together by mechanical pressing in place of spraying water jet to produce each laminated non - woven fabric . the thickness , average pore size , initial efficiency , and life of each laminated non - woven fabric are shown in table 2 . table 2______________________________________ compar - compar - compar - compar - ative ative ative ative example example example example 1 2 3 4______________________________________production conditionsnon - woven fabricfirst layer ppnw - 1 ppnw - 1 ppnw - 2 ppnw - 2second layer ppnw - 2 ppnw - 3 ppnw - 3 ppnw - 3third layer ppnw - 3 ppnw - 4 ppnw - 4 -- fourth layer ppnw - 4 -- -- -- propertiesthickness ( mm ) 1 . 90 1 . 85 1 . 90 0 . 72gas permeability 4 . 5 6 . 2 7 . 1 11 . 3 ( cc / cm . sup . 2 / sec ) average pore size 14 16 17 17 ( μm ) initial efficiency (%) 72 55 38 35life ( cc · 2 kgf / cm . sup . 2 ) 800 800 1200 1220______________________________________ as can be seen from the above results , the laminated non - woven fabrics of examples 1 - 4 showed good initial efficiency and high values of life as compared with the corresponding laminated non - woven fabrics of comparative examples 1 - 4 . such laminated non - woven fabrics of the present invention are suitable for use in various types of filter such as electric battery separator , heat - resistant filter , etc .