A novel product operator formalism for strongly coupled two-spin systems was developed which allows the time evolution for such systems to be estimated much more easily than using the density matrix. The newly developed theory was applied to describe the INADEQUATE (Incredible Natural Abundance DoublE QUAntum Transfer Experiment) experiments, and the condition for the efficiency of the creation of the double-quantum coherences was formulated, which is useful for setting the experimental conditions. The refocused INADEQUATE spectra of benzyl bromide, in which two one-bond13C—13C pairs have extremely strong couplings compared with the chemical shift differences, were observed under several experimental conditions. The signals attributable to these strongly coupled spin pairs were detected with reasonably large intensities, and their coupling constants were determined by simulation of the spectral intensities; the weak and the long-range couplings in the compound were also obtained.