A simple monomeric tungstate, TBA(2)[WO4] (I, TBA = tetra-n-butylammonium), could act as an efficient homogeneous catalyst for chemical fixation of CO2 with 2-aminobenzonitriles to quinazoline-2,4(1H,3H)-diones. Various kinds of structurally diverse 2-aminobenzonitriles could be converted into the corresponding quinazoline-2,4(1H,3H)-diones in high yields at atmospheric pressure of CO2. Reactions of inactive 2-amino-4-chlorobenzonitrile and 2-amino-5-nitrobenzonitrile at 2 MPa of CO2 also selectively proceeded. The present system was applicable to a g-scale reaction of 2-amino-5-fluorobenzonitrile (10 mmol scale) with CO2 and 1.69 g of analytically pure quinazoline-2,4(1H,3H)-dione could be isolated. In this case, the turnover number reached up to 938 and the value was the highest among those reported for base mediated systems so far. NMR spectroscopies showed formation of the corresponding carbamic acid through the simultaneous activation of both 2-aminobenzonitirile and CO2 by I. Kinetic and computational studies revealed that I plays an important role in conversion of the carbamic acid into the product.