Technical Field The present invention relates to a Mo-Bi-Nb-Te based composite metal oxide, and a process for producing (meth) acrylic acid from propylene or the like by using the Mo-Bi-Nb-Te, based composite metal oxide as a catalyst Also, the present invention relates to a process for producing (meth)acrylic acid comprising a first step of producing (meth)acrolein as a main product from propylene or the like, and a second step of producing (meth)acrylic acid from the (meth)acrolein, wherein the yield of (meth)acrylic acid in the product of the first step is 20 mole% or higher Background Art A process for producing an unsaturated fatty acid from an olefin by way of an unsaturated aldehyde is a typical process of catalytic vapor phase oxidation To perform partial oxidation of olefins, composite oxides containing molybdenum and bismuth, molybdenum and vanadium, or mixtures thereof are used as catalysts Particular examples of such catalytic vapor phase oxidation include a process of producing (meth)acryliu acid by the oxidation of propylene or isobutylene by way of (meth)acrolein, a process of producing phthalic anhydride by the oxidation of naphthalene or orthoxylene, and a process of producing maleic anhydride by the partial oxidation of benzene, butylene or butadiene Generally, (meth)acrylic acid, a final product, is produced from at least one reaction material selected from the group consisting of propylene, propane, isobutylene, t-butyl alcohol or methyl-t-butyl ether (referred to as 'propylene or the like' , hereinafter) by a two-step process of vapor phase catalytic partial oxidation More particularly, in the first step, propylene or the like is oxidized by oxygen, inert gas for dilution, water steam and a certain amount of a catalyst, so as to produce (meth) acrolein as a main product Then, in the second step, the (meth)acrolein is oxidized by oxygen, inert gas for dilution, water steam and a certain amount of a catalyst, so as to produce (meth)acrylic acid The catalyst used in the first step is a Mo-Bi-based multinary metal oxide, which oxidizes propylene or the like to produce (meth)acrolein as a mam product Also, some acrolein is continuously oxidized on the same catalyst to partially produce (meth)acrylic acid The catalyst used in the second step is a Mo-V-based multinary metal oxide, which mainly oxidizes (meth)acrolein in the mixed gas containing the (meth)acrolein produced from the first step to produce (meth)acrylic acid as a main product A reactor for performing the aforementioned process is provided either in such a manner that both the two-steps can be performed in one system, or in such a manner that the two steps can be performed in different systems As mentioned hereinbefore, tne first-step catalyst involved in vapor phase partial oxidation using propylene or the like as a starting material is a multinary metal oxide, with which (meth)acrolein is produced as a main product and at most 10% of (meth)acrylic acid is produced As disclosed in Japanese Laid-Open Patent No Hei8-3093, a conventional first-step catalyst is a composite oxide represented by the formula of Moa-B1b- Fec-Ad-Be-Cf-Dg-Ox, wherein Mo, Bi and Fe represent molybdenum, bismuth and iron, respectively, A is nickel and/or cobalt, B is at least one element selected from the group consisting of manganese, zinc, calcium, magnesium, tin and lead, C is at least one element selected from the group consisting of phosphorus, boron, arsenic, Group 6B elements in the Periodic Table, tungsten, antimony and silicon, D is at least one element selected from the group consisting of potassium, rubidium, cesium and thallium, each of a, b, c, e, f and g is a number satisfying the conditions of 0 Preparation Example 1 Catalyst 1 First, 2500 ml of distilled water was heated and stirred at 70°C~ 85°C and lOOOg of ammonium molybdate was added thereto to form solution (1) Next, 274g of bismuth nitrate, 228g of ferrous nitrate and 2 3g of potassium nitrate were added~ to 400ml of distilled water, the materials were mixed thoroughly, 71g of nitric acid was added thereto, and the materials were dissolved sufficiently to form solution (2) To 200ml of distilled water, 686g of cobalt nitrate was dissolved to form solution (3) After mixing solution (2) with solution (3) , the mixed solution was further mixed with solution (1) while maintaining the temperature at 40- 60°C to provide a catalyst suspension The catalyst suspension was dried to produce M012B11 2Fei 2C05K0 05 and the catalyst was pulverized into a size of 150^m or less The resultant catalyst powder was mixed for 2 hours and formed into a cylindrical shape The catalyst was formed to have an outer diameter of 4 0-8 0mm, and calcined at 500°C for 5 hours under the aj.r, and then the catalytic activity was verified Preparation Example 2 Catalyst 2 Catalyst 2 was provided in the same manner as described in Preparation Example 1, except that 63g of niobium chloride and 150g of tellurium chloride were further added to form solution (1) The catalyst had the elemental composition of M012ND0 5TeiBii 2Fei 2C05K0 05 except oxygen Preparation Example 3 Catalyst 3 Catalyst 3 was provided in the same manner as described in Preparation Example 1, except that 127g of niobium chloride and 150g of tellurium nitrate were further added to form solution (1) The catalyst had the elemental composition of Moi2Nbi 0Tei 0Bii 2Fei 2Co4 5K0 05 except oxygen Preparation Example 4 Catalyst 4 Catalyst 4 was provided in the same manner as described in Preparation Example 1, except that 63g of niobium chloride and 75g of tellurium chloride were further added to form solution (1) The catalyst had the elemental composition of Moi2Nbo sTeiBii 2?ei 2C04 5K0 05 except oxygen Preparation Example 5 Catalyst 5 First, 2000 ml of distilled water was heated and stirred at 100°C and 246g of ammonium tungstate, lOOOg of ammonium molybdate and 220g of ammonium vanadate were dissolved therein to form solution (1) Next, 228g of copper nitrate and 4 9g of strontium nitrate were added to 500ml of distilled water, and the materials were mixed thoroughly to form solution (2) Solution (1) was mixed with solution (2) to provide a suspension The suspension was treated by using a homogenizer for at least 30 minutes and was coated- on spherical- carriers having an outer diameter of 4 0-8 0mm by using a spray nozzle to an amount of 20- 30 wt% as expressed by the catalytically active component present in the suspension The coated catalyst was dried at 120°C sufficiently and calcined at 400°C for at least 5 hours to provide spherical catalyst particles having a final outer diameter of 5mm{±0 2) The catalyst had the elemental composition of M012W2 0V4 0CU2 oSr0 5 except oxygen Experiment Catalyst Packing and Catalytic Activity Test> To a 3m stainless steel reactor having an inner diameter of 1 inch and heated with molten nitrate salt, alumina silica was packed to a height of 150mm as an inert material, and any one of Catalysts 1~ 4 was packed to a height of 2800mm as the first-step catalyst, from the inlet of the reaction gas toward the outlet Then, alumina silica was packed to a height of 150mm as an inert material and Catalyst 5 was packed to a height of 2900mm as the second-step catalyst Propylene was subjected to vapor phase oxidation by using the reactor to produce acrolein and acrylic acid The first-step oxidation was performed by introducing feed gas containing 7 vol% of propylene, 13 vol% of molecular oxygen, 8 vol% of water steam and 72 vol% of inert gas onto the catalyst with a space velocity of 1500 hr"1 (STP) , at a reaction temperature of 320°C, under a reaction pressure of 0 7 atm The second-step oxidation was performed at a reaction temperature of 276°C, under a reaction pressure of 0 1~ 3kg/cm2G In the following Tables 1 and 2, conversion ratio of a reaction material, selectivity and yield are calculated based on the following Mathematical Formulae 1~ 7 [ Mathematical Formula 1] first-step propylene conversion ratio(%) = [ moles of reacted propylene/moles of supplied propylene] X 100 [ Mathematical Formula 2] yield(%) of acrolein in the first step = [moles of produced acrolein/moles of supplied propylene] X 100 [ Mathematical Formula 3] yield(%) of acrylic acid in the first step= [moles of produced acrylic acid/moles of supplied propylene] X 100 [ Mathematical Formula 4] selectivity(%) of acrolein + acrylic acid in the first step = [ moles of produced acrolein and acrylic acid/moles of reacted propylene] X 100 [ Mathematical formula 5] second-step acrolein conversion ratio(%) = [moles of reacted acrolein/moles of supplied acrolein] X 100 [ Mathematical Formula 6] yield(%) of acrylic acid in the second step= [ moles of produced acrylic acid/moles of supplied acrolein] X 100 [ Mathematical Formula 7] selectivity(%) of acrylic acid in the second step = [ moles of produced acrylic acid/moles of reacted acarolein] X 100 The experimental results of the Examples according to the present invention and Comparative Example are shown in the following Table 1 (first-step oxidation) and Table 2 {second-step oxidation) Industrial Applicability As can be seen from the foregoing, when the Mo-Bi- Nb-Te based composite metal oxide according to the present invention is used as the first-step catalyst in the production of (meth)acrylic acid from propylene or the like, yield and/or selectivity of (meth)acrylic acid increases in the first-step reaction product, and thus (meth)acrolein load decreases in the second-step to such a degree that (meth*) acrolein conversion ratio can reach 98- 100% While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment and the drawings On the contrary. it is intended to cover various modifications and variations within the spirit and scope of the appended claims WE CLAIM 1 A process for producing (meth) acrylic acid comprising a first step of producing (meth) acrolein as a main product from at least one reaction material selected from the group consisting of propylene, isobutylene, t-butyl alcohol and methyl-t-butyl ether, and a second step of producing (meth) acrylic acid from the (meth) acrolein, wherein yield of (meth)acrylic acid in the product of the first step is 20 mole % or higher, wherein a Mo-Bi-Nb-Te based composite metal oxide is used as a catalyst of the first step and the composite metal oxide is represented by the following Formula 1 [Formula 1 ] MOaB1bNbcTedFefCogKJOk Wherein Mo represents molybdenum, Bi represents bismuth, Nb represents niobium, Te represents tellurium, Fe represents iron, Co represents cobalt, K represents potassium, and O represents oxygen, each of a, b, c, d, f, g, j and k represents the atomic ratio of each element, wherein when a= 12, b is 0 01-20, c is 0 001-20, d is 0 001-20, f is 1 2, g is 4 5 or 5, J is 0 05, and k is a number defined by the oxidation state of each of the above elements 2 The process as claimed in claim 1, wherein the first-step reaction product includes (meth) acrolein and (meth) acrylic acid in a molar ratio ((meth)acrolein (meth) acrylic acid) of 8 2-7 3 3 The process as claimed in claim 1, wherein conversion ratio of (meth) acrolein in the second step is 98% - 100% 4 The process as claimed in claim 1, wherein the reaction material introduced into the first step comprises at least one reaction material selected from the group consisting of propylene, isobutylene, t-butyl alcohol and methyl-t-butyl ether in a concentration of 7-10 vol % ABSTRACT PROCESS FOR PRODUCTION OF (METH) ACRYLIC ACID The invention is for a process for producing (meth)acrylic acid comprising a first step of producing (meth)acrolein as a main product from at least one reaction material selected from the group consisting of propylene, isobutylene, t-butyl alcohol and methyl-t-butyl ether, and a second step of producing (meth)acrylic acid from the (meth)acrolein, wherein yield of (meth)acrylic acid in the product of the first step is 20 mole % or higher, wherein a Mo—B1—Nb—Te based composite metal oxide is used as a catalyst of the first step and the composite metal oxide is represented by the following Formula 1 [Formula 1] MOaB1bNbcTedFefCOgKJOI Wherein Mo represents molybdenum, B1 represents bismuth, Nb represents niobium, Te represents tellurium, Fe represents iron, Co represents cobalt, K represents potassium, and 0 represents oxygen, each of a, b, c, d, f, g, j and k represents the atomic ratio of each element, wherein when a=12, b is 0 01-20, c is 0 001-20, d is 0 001-20, f is 1 2 g is 4 5 or 5, j is 0 05, and k is a number defined by the oxidation state of each of the above elements