Vibrational structure of the pyridazine cation in the ground state has been revealed by a vacuum-ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy. The adiabatic ionization energy is precisely measured to be 70241 +/- 6 cm(-1) (8.7088 +/- 0.0007 eV). The origin is very weakly observed, while a long progression of the nu(+)(9) (a(1)) band of which the fundamental vibrational frequency is 647 cm(-1) is predominantly observed. The nu(+)(9) (a(1)) mode progression combined with one quantum of the nu(+)(3) (a(1)) band at 1698 cm(-1) is found to be even stronger. Many other weakly observed vibrational features of the pyridazine cation are identified in the vibrational energy of 0-3500 cm(-1). The structural change of pyridazine upon ionization, reflected in the vibrational spectrum obtained by the one-photon direct ionization process, is theoretically predicted by ab initio calculations. Ring distortion including contraction of the N=N bond should be responsible for strong excitations of nu(+)(3) and nu(+)(9) modes. Franck-Condon analysis is given for the comparison of the experiment and theory.