[DESCRIPTION] [Invention Title] MULTI-METAL OXIDE CATALYST AND METHOD FOR PRODUCING (METH)ACRYLIC ACID BY USING THE SAME [Technical Field] The present invention relates to a Mo-Bi-Nb based composite metal oxide(multi-metal oxide), and a method for producing (meth)acrylic acid from propylene or the like by using the Mo-Bi-Nb based composite metal oxide as a catalyst. Further, the present invention relates to a method for producing (meth)acrylic acid from propylene or the like by an one-step catalyst reaction. This application claims priority from Korea Patent Application No. 10-2006-71061 filed on July 27, 2006 in the KIPO, the disclosure of which is incorporated herein by reference in its entirety. [Background Art] A process for producing an unsaturated fatty acid from olefin by way of an unsaturated aldehyde is a typical process of gas-phase catalytic oxidation. Particular examples thereof include a process of producing (meth)acrylic acid from a staring material such as propylene, propane, isobutylene, t-butyl alcohol or methyl-t-butyl ether (referred to as 'propylene or the like', hereinafter) by way of corresponding (meth)acrolein. In this connection, in the first step of partially oxidizing olefins to unsaturated aldehyde, 1 composite metal oxides containing molybdenum and bismuth are generally used as a catalyst. In the second step of partially oxidizing the unsaturated aldehyde, which is a main product of the first step, to unsaturated fatty acid, composite metal oxides containing molybdenum and vanadium are used as a catalyst. 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 main product. Also, some (meth)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 of 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 above, the first-step catalyst involved in gas-phase partial oxidation using propylene or the like as a starting material is the Mo-Bi based multi-metal oxide, with which (meth)acrolein is produced as a main product and 10% or less of (meth)acrylic acid is produced. 2 As disclosed in JP-A-8-3093, a conventional first-step catalyst is a composite oxide represented by the formula of Moa-Bib-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; when a=12, 0 Preparation Example 1: Catalyst 1 2500 ml of distilled water was heated and stirred at 70 to 85 °C and 1000 g of ammonium molybdate was dissolved therein to form a solution 1. Then, 274 g of bismuth nitrate, 228 g of ferrous nitrate and 2.3 g of potassium nitrate were added to 400 ml of distilled water, the materials were mixed thoroughly, 71 g of nitric acid was added thereto, and the materials were dissolved sufficiently to form a solution 2. 686 g of cobalt nitrate was dissolved in 200 ml of distilled water, so as to form a solution 3. After mixing the solution 2 with the solution 3, the mixed solution was further mixed with the solution 1 while the temperature was maintained at 40 to 60 °C, so as to provide a catalyst suspension. 14 The catalyst suspension was dried and the obtained cake-shaped solid was pulverized into a size of 150 μm or less. The resultant catalyst powder was mixed with a predetermined amount of water for 2 hours, and formed into a cylindrical shape. The catalyst was formed to have a diameter of 5.0 mm and a height of 5.0 mm, and calcined at 5001) for 5 hours under the air, resulting in a catalyst 1. The produced catalyst had the elemental composition of except oxygen. The resulting catalyst had the following elemental composition except oxygen: Mo12 Bi1..2 Fe1..2 CO5 K0.05 Preparation Example 2: Catalyst 2 Catalyst 2 was provided in the same manner as described in Preparation Example 1, except that 64 g of niobium chloride were further added to form a solution 1. The resulting catalyst had the following elemental composition except oxygen: Mo12 Nb0.5 Bi1..2 Fe1..2 C05 K0.05 Preparation Example 3: Catalyst 3 Catalyst 3 was provided in the same manner as described in Preparation Example 1, except that 64 g of niobium chloride were further added to form a solution 1 and the molded catalyst was allowed to have a diameter of 7 mm and a height of 7 mm. The resulting catalyst had the following elemental composition except oxygen: Mo12 Nb0.5 Bi1..2 Fe1..2 C05 K0.05 Preparation Example 4: Catalyst 4 Catalyst 4 was provided in the same manner as described in Preparation Example 1, 15 except that 32 g of niobium chloride were further added to form a solution 1. The resulting catalyst had the following elemental composition except oxygen: Mo12 Nb0.5 Bi1..2 Fe1..2 C05 K0.05 Preparation Example 5: Catalyst 5 Catalyst 5 was provided in the same manner as described in Preparation Example 1, except that 32 g of niobium chloride were further added to form a solution 1 and the molded catalyst was allowed to have a diameter of 7 mm and a height of 7 mm. The resulting catalyst had the following elemental composition except oxygen: Mo12 Nb0.5 Bi1..2 Fe1..2 C05 K0.05 To a 3 m stainless steel reactor having an inner diameter of 1 inch and heated with molten nitrate salt, alumina silica was packed to a height of 150 mm as an inert material, and any one or a mixture of Catalysts 1 to 5 prepared in Catalyst Preparation Examples 1 to 5 shown in Table 1 was packed to have a height of 2800 mm, from the inlet of the reaction gas toward the outlet. The oxidation was performed by introducing feed gas containing 7 vol% of propylene, 13 vol% of oxygen, 12 vol% of water steam, and 68 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 arm. 16 In Tables 1, conversion ratio of a reactant and yield are calculated, based on the following Mathematical Formulae 1 and 2. [Mathematical Formula 1] Conversion ratio of propylene (%) = [(mole number of reacted propylene)/(mole number of supplied propylene)] x 100 [Mathematical Formula 2] Yield (%) of acrylic acid = [(mole number of produced acrylic acid)/(mole number of supplied propylene)] x 100 The experimental results of the Examples and Comparative Example are shown in the following Table 1. [Table 1] Section Comparative 1 Example 1 Example 2 Example 3 Catalyst packed Catalyst 1 (2800mm) Catalyst 2 (2800mm) Catalyst 3 (800mm) + Catalyst 2 (2000mm) Catalyst 5 (800mm) + Catalyst 4 (2000mm) Conversion ratio of propylene (%) 3201 98.67 92.21 91.44 93.87 Yield (mole %) of acrylic acid 9.51 67.40 72.65 76.84 We claim: [Claim 1] A reactor for producing (meth)acrylic acid using one or more reactant selected from the group consisting of propylene, propane, isobutylene, t-butyl alcohol and methyl-t-butyl 5 ether, the reactor comprising an inlet, an outlet, and a reaction zone, wherein the reaction zone includes a catalyst of a Mo-Bi-Nb based composite metal oxide, the catalyst being represented by the following Formula 1: [Formula 1] MOa Bib Nbc Ad Be Cf Dg Eh F| Oj 10 wherein Mo represents molybdenum, Bi represents bismuth, and Nb represents niobium; A is one or more element selected from the group consisting of W, Sb, As, P, Sn and Pb; B is one or more element selected from the group consisting of Fe, Zn, Cr, Mn, Cu, Pd, 15 Ag and Ru; C is one or more element selected from the group consisting of Co, Cd, Ta, Pt and Ni; D is one or more element selected from the group consisting of Si, Al, Zr, V and Ce; E is one or more element selected from the group consisting of Se, Ga, Ti, Ge, Rh and Au; 20 F is one or more element selected from the group consisting of Na, K, Li, Rb, Cs, Ca, Mg, Sr, Ba and MgO; each of a, b, c, d, e, f, g, h, i, and j represents the atomic ratio of each element; and when a=12, b is 0.01 to 20, c is 0.001 to 20, d is 0 to 15, e is 0 to 20, f is 0 to 20, g is 0 to 10, h is 0 to 10, i is 0 to 10, and j is a value defined by the oxidation state of each of the 25 above elements, proviso that g is zero and Te is not included. [Claim 2] The reactor as claim in claim 1, wherein the reactor is a shell and tube type reactor, wherein the shell and tube type reactor further comprises a tube that includes the 30 reaction zone, and wherein the reaction zone of the tube is packed with the Mo-Bi-Nb based composite metal oxide catalyst. 18 10 '^}M [Claim 3] The reactor - as claim in claim 2, wherein the reaction zone of the tube includes two or more catalyst beds based on the Mo-Bi-Nb based composite metal oxide. [Claim 4] The reactor as claim in claim 3, wherein the reaction zone of the tube includes two or more of catalyst beds having different catalytic activities in order to increase the catalytic activity of the catalyst bed from the inlet, in which the reactants are introduced, to the outlet, in which the reaction products are outputted, along its tubular axis. [Claim 5] The reactor as claim in claim 2, wherein the reaction zone of the tube includes two or more different catalyst beds so that the particle size of the catalyst in the catalyst beds packed with Mo-Bi-Nb based composite metal oxide decreases from the inlet, in which the 15 reactants are introduced, to the outlet, in which the reaction products are outputted, along its tubular axis. [Claim 6] The reactor as claim in claim 4, wherein the reaction zone of the tube includes two or 20 more different catalyst beds packed with Mo-Bi-Nb based composite metal oxide and the different molar ratios of Nb to Mo ([Nb]/[Mo]) and the catalytic activity of each of the catalyst beds increases from the inlet, in which the reactants are introduced, to the outlet, in which the reaction products are outputted, along its tubular axis. 25 [Claim 7] The reactor as claim in claim 1, wherein the reactor is a fixed-bed reactor, a fluidizedbed reactor or a moving-bed reactor. [Claim 8] 30 The reactor as claim in claim 1, wherein the catalyst has a yield of 60 mol% or more 19 ( when used as a single catalyst. [Claim 9] The reactor as claim in claim 7, wherein the reactor is a fluidized-bed reactor, and wherein a mixture of solid catalyst and reaction gas moves through the inlet to the reaction zone to the reaction zone, and after reaction, non-reactants, products and the catalyst move though the outlet in a mixed state.