The development of colorimetric and fluorescence chemical sensors that have a function of in situ selective sensing of biologically important anions is one of the significant issues in host-guest chemistry, and a large number of studies have been carried out in the field. The fluorescent chemical sensor, N-(4-Dimethylaminobenzoyl)thiourea (DMABTU; Chart 1) that has been reported by Wu etal. [1], has two emission wavelengths at ca. 500nm (stronger) and ca. 350nm (weaker)) in the absence of anions in chloroform solution. By the addition of anions such as acetate, the emission band at 500nm decreases while the emission band at 350nm increases. The broad emission band at around 500nm was assigned to an intramolecular charge transfer (ICT) transition, and the narrow emission band at around 350nm to a locally excited (LE) transition. However, a detailed theoretical analysis has not been performed yet on the turn-off switching mechanism of ICT induced by the anion recognition. Therefore, in this work, to clarify the ICT turn-off switching mechanism, we carried out a density functional theory (DFT) and time-dependent DFT (TDDFT) calculations on DMABTU in the ground (S-0) and excited (S-1) states. We obtained optimized structures of DMABTU-X-(X- = free, acetate) in S-0 and S-1, and also obtained potential-energy curves (PEC) with respect to a dihedral angle delta between the dimethylamino-group and phenyl-group planes. From the calculations, it was proved that the PEC of DMABTU is not quadratic but nearly flat between delta = 0 similar to 10 degrees, and the potential minimums are located at delta = ca. 8 similar to 9 degrees both in S-0 and S-1, while the PEC of DMABTU-AcO- is quadratic, and DMABTU-AcO- takes twisted structures (delta = ca. 12 degree in S-0 and delta = ca. 21 degree in S-1). These results suggest that ICT character of DMABTU is not a twisted ICT (TICT) but a planar ICT (PICT) state.