Sub-Doppler high-resolution excitation spectra of dibenzo-p-dioxin (DD) have been observed for the 0(0)(0) and 0(0)(0) + 22 cm(-1) bands of the S-1 <-- S-0 transition by crossing a tunable laser beam perpendicular to a collimated 0 molecular beam. The spectrum was composed of several strong Q lines at the band center and P and R lines on both sides. The rotational constants of the S-0(v" = 0), S-1 (v' = 0), and S-1 (v' = 0 + 22 cm(-1) ) states were determined by analysis of the observed rotational contours. The observed spectra were reproduced well by simulating the spectra as the a-type transition, whose transition moment is parallel to the long axis of the molecule. Accordingly, the S-1 state is identified to be a B-1(1u) state. From the rotational constants, the DD molecule has been shown to be planar in the S-0 state and folded slightly out-of-plane (butterfly form) in the S-0 state: the dihedral angles of the phenyl ring are 170.6degrees and 173.4degrees in the S-1(v' = 0) and S-1(v' = 0 + 22 cm(-1)) states, respectively. The 0(0)(0) + 22 cm(-1) band is assigned as a transition to the a(g) component of the butterfly mode. The line widths were much larger than the instrumental resolution, and the lifetime of the S-1 state was evaluated to be similar to500 ps.