[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, 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. 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. 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, 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. 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] (Table Removed) [CLAIMS] [Claim 1] A Mo-Bi-Nb based composite metal oxide (with the proviso that Te is not included). [Claim 2] The Mo-Bi-Nb based composite metal oxide according to claim 1, which is represented by the following Formula 1: [Formula 1] (Formula Removed) wherein Mo represents molybdenum, Bi represents bismuth, and Nb represents niobium; A is at least one element selected from the group consisting of W, Sb, As, P, Sn and Pb; B is at least one element selected from the group consisting of Fe, Zn, Cr, Mn, Cu, Pd, Ag and Ru; C is at least one element selected from the group consisting of Co, Cd, Ta, Pt and Ni; D is at least one element selected from the group consisting of Si, Al, Zr, V and Ce; E is at least one element selected from the group consisting of Se, Ga, Ti, Ge, Rh and Au; F is at least one 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 above elements. [Claim 3] The Mo-Bi-Nb based composite metal oxide according to claim 1 or 2, which is used as a catalyst. [Claim 4] The Mo-Bi-Nb based composite metal oxide according to claim 3, which is used as a catalyst of gas-phase catalytic partial oxidation. [Claim 5] A method for producing (meth)acrylic acid using at least one reactant selected from the group consisting of propylene, propane, isobutylene, t-butyl alcohol and methyl-t-butyl ether, wherein a Mo-Bi-Nb based composite metal oxide (with the proviso that Te is not included) is used as a catalyst. [Claim 6] The method according to claim 5, wherein a yield of (meth)acrylic acid of products obtained by catalytic action of the Mo-Bi-Nb based composite metal oxide is 60 mole% or more. [Claim 7] The method according to claim 5, wherein the method is performed in a fluidized bed reactor. [Claim 8) The method according to claim 5, wherein the Mo-Bi-Nb based composite metal oxide is represented by the following formula 1: [Formula 1] (Formula Removed)wherein Mo represents molybdenum, Bi represents bismuth, and Nb represents niobium; A is at least one element selected from the group consisting of W, Sb, As, P, Sn and Pb; B is at least one element selected from the group consisting of Fe, Zn, Cr, Mn, Cu, Pd, Ag and Ru; C is at least one element selected from the group consisting of Co, Cd, Ta, Pt and Ni; D is at least one element selected from the group consisting of Si, Al, Zr, V and Ce; E is at least one element selected from the group consisting of Se, Ga, Ti, Ge, Rh and Au; F is at least one 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 above elements. [Claim 9] The method according to claim 5, further comprising a process of converting (meth)acrolein of the products produced by the catalytic action of the Mo-Bi-Nb based composite metal oxide into (meth)acrylic acid. [Claim 10] The method according to claim 5, wherein a reaction zone in which (meth)acrylic acid is produced using at least one reactant selected from the group consisting of propylene, propane, isobutylene, t-butyl alcohol and methyl-t-butyl ether is packed with two or more catalyst beds, and the Mo-Bi-Nb based composite metal oxide is used as a catalytic effective component of at least one catalyst bed. [Claim 11] The method according to claim 10, wherein the reaction zone is packed with 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. [Claim 12] The method according to claim 10, wherein the reaction zone is packed with two or more different catalyst beds so that the particle size of the catalyst in the catalyst beds having an effective component of Mo-Bi-Nb based composite metal oxide decreases from the inlet, in which the reactants are introduced, to the outlet, in which the reaction products are outputted. [Claim 13] The method according to claim 11, wherein the reaction zone is packed with two or more different catalyst beds having an effective component of 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. [Claim 14] A method for producing (meth)acrylic acid using at least one reactant selected from the group consisting of propylene, propane, isobutylene, t-butyl alcohol and methyl-t-butyl ether by using a Mo-Bi-Nb based composite metal oxide catalyst, without any additional process of converting (meth)acrolein into (meth)acrylic acid. [Claim 15] The method according to claim 14, wherein the method is performed in a fluidized bed reactor. [Claim 16] The method according to claim 14, wherein a reaction zone in which (meth)acrylic acid is produced using at least one reactant selected from the group consisting of propylene, propane, isobutylene, t-butyl alcohol and methyl-t-butyl ether is packed with two or more catalyst beds, and the Mo-Bi-Nb based composite metal oxide is used as a catalytic effective component of at least one catalyst bed. [Claim 17] The method according to claim 16, wherein the reaction zone is packed with 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. [Claim 18] The method according to claim 16, wherein the reaction zone is packed with two or more different catalyst beds so that the particle size of the catalyst in the catalyst beds having an effective component of Mo-Bi-Nb based composite metal oxide decreases from the inlet, in which the reactants are introduced, to the outlet, in which the reaction products are outputted. [Claim 19] The method according to claim 17, wherein the reaction zone is packed with two or more different catalyst beds having an effective component of 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. [Claim 20] A reactor for producing (meth)acrylic acid using at least one reactant selected from the group consisting of propylene, propane, isobutylene, t-butyl alcohol and methyl-t-butyl ether, wherein a Mo-Bi-Nb based composite metal oxide is used as a catalyst. [Claim 21] The reactor according to claim 20, wherein the reactor is a shell and tube type reactor and the tube is packed with the Mo-Bi-Nb based composite metal oxide catalyst. [Claim 22] The reactor according to claim 21, wherein a reaction zone of the tube in which (meth)acrylic acid is produced using at least one reactant selected from the group consisting of propylene, propane, isobutylene, t-butyl alcohol and methyl-t-butyl ether is packed with two or more catalyst beds, and the Mo-Bi-Nb based composite metal oxide is used as a catalytic effective component of at least one catalyst bed. [Claim 23] The reactor according to claim 22, wherein the reaction zone is packed with 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 24] The reactor according to claim 21, wherein the reaction zone of the tube is packed with two or more different catalyst beds so that the particle size of the catalyst in the catalyst beds having an effective component of Mo-Bi-Nb based composite metal oxide decreases from the inlet, in which the reactants are introduced, to the outlet, in which the reaction products are outputted, along its tubular axis. [Claim 25) The reactor according to claim 23, wherein the reaction zone of the tube is packed with two or more different catalyst beds having an effective component of 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.