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Dibenzofuran is one of the prototypical molecule of toxic dioxins and it is great important to investigate the excited-state dynamics. We have observed low-resolution spectra for the vibronic structure of the $S11A1\leftarrow S01A1$ transition of jet-cooled dibenzofuran. Several vibronic bands are stronger than the 0$\mathrm{00}$ band. These bands are found to be the $A$-type transition which are enhanced by intensity borrowing through the vibronic coupling with the $S21B2$ state. } {\bf 110}, 10000 (2006).} Rotationally resolved ultrahigh-resolution spectra of jet-cooled dibenzofuran for the 0$\mathrm{00}$ and 0$\mathrm{00}$+443 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ band have been observed. Several thousand lines were assigned and these molecular constants were determined. }, {\bf 238}, 49 (2006).} We have extended the high-resolution spectroscopic measurement for several vibronic bands up to 1335 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. We analyzed observed rotational lines and determined the molecular constants for each vibronic bands. The Zeeman broadenings of the rotational lines were also observed, and the $J$, $K$-dependence were studied. As a result, the observed Zeeman effects could be explained to be originating from the magnetic moment of the $S11A1$ state induced by mixing with $S21B2$ state by $J$-$L$ coupling.