In our Galaxy, molecular gas forms clouds with masses of ~ 105M© and with sizes of several 10 pc, which are called Giant Molecular Clouds (Sanders 1985; hereafter GMCs). Such GMCs are also observed in nearby spiral galaxies M31 and M33 (Lada et al. 1988; Wilson & Scoville 1990). On the other hand, one of the well-studied spiral galaxy M51 has more massive and larger molecular clouds. Since their typical mass and size are 107-8MQ and several 100 pc, respectively, they are called Molecular Superclouds (Rand & Kulkarni 1990; Tosaki et al. 1992; hereafter, MSCs). A question arises as what causes the difference in structural properties of molecular gas among the spiral galaxies. In order to answer this question, we should make higher resolution studies both spatially and spectroscopically. In this paper, we present the results of high spectral-resolution study of the MSCs in M51 based on the 12CO (J = 1 − 0) mapping made with the Nobeyama Millimeter Array (Tosaki et al. 1992). The spatial resolution is 5.”3×4.”9 corresponding to 240 × 230 pc at distance of 9.6 Mpc. The spectral resolution is 3.25 km s−1. The analysis is made for the 1’ region whose center is offset 42” south and 7” east from the nucleus, because a very intense molecular arm is present in this region. Our Main results and conclusions are as follows.