  We present the results of the CO mapping observations of the barred spiral
galaxy NGC3504 with the Nobeyama 45-m telescope. The distribution of the
molecular gas shows offset ridges which correspond to the distribution of HII
regions along the bar. The velocity perpendicular to the bar decreases abruptly
at the ridge. The velocity change implies that the molecular gas changes the
direction of its motion to parallel to the bar at the ridge. Since the position
angle of the major axis of the bar and the line of nodes are almost the same in
NGC 3504, an upper limit to the pattern speed of the bar can be derived
directly from the radial velocity. The resultant upper limit is 41 km/s/kpc
which is much smaller than that derived with an assumption that the corotation
radius is located at the end of the bar (77 km/s/kpc). The corotaion radius
derived from our upper limit is more than two times larger than the length of
the semi-major axis of the bar in NGC 3504.
