  A survey of molecular cores covering the infrared dark cloud known as the M17
southwest extension (M17 SWex) has been carried out with the 45 m Nobeyama
Radio Telescope. Based on the N2H+ (J= 1-0) data obtained, we have identified
46 individual cores whose masses are in the range 43 to 3026 Mo. We examined
the relationship between the physical parameters of the cores and those of
young stellar objects (YSOs) associated with the cores found in the literature.
The comparison of the virial mass and the core mass indicates that most of the
cores can be gravitationally stable if we assume a large external pressure.
Among the 46 cores, we found four massive cores with YSOs. They have large mass
of >~ 1000Mo and line width of >~ 2.5 km s^-1 which are similar to those of
clumps forming high mass stars. However, previous studies have shown that there
is no active massive star formation in this region. Recent measurements of
near-infrared polarization infer that the magnetic field around M17 SWex is
likely to be strong enough to support the cores against self-gravity. We
therefore suggest that the magnetic field may prevent the cores from
collapsing, causing the low-level of massive star formation in M17 SWex.
