  The main accretion phase of star formation is investigated in clouds with
different metallicities in the range of 0 \le Z \le Z_\odot, resolving the
protostellar radius. Starting from a near-equilibrium prestellar cloud, we
calculate the cloud evolution up to \sim100 yr after the first protostar
formation. The star formation process considerably differs between clouds with
lower (Z \le 10^-4 Z_\odot) and higher (Z > 10^-4 Z_\odot) metallicities.
Fragmentation frequently occurs and many protostars appear without forming a
stable circumstellar disc in lower-metallicity clouds. In these clouds,
although protostars mutually interact and some are ejected from the cloud
centre, many remain as a small stellar cluster. In contrast, higher-metallicity
clouds produce a single protostar surrounded by a nearly stable
rotation-supported disc. In these clouds, although fragmentation occasionally
occurs in the disc, the fragments migrate inwards and finally fall onto the
central protostar. The difference in cloud evolution is due to different
thermal evolutions and mass accretion rates. The thermal evolution of the cloud
determines the emergence and lifetime of the first core. The first core
develops prior to the protostar formation in higher-metallicity clouds, whereas
no (obvious) first core appears in lower-metallicity clouds. The first core
evolves into a circumstellar disc with a spiral pattern, which effectively
transfers the angular momentum outwards and suppresses frequent fragmentation.
In lower-metallicity clouds, the higher mass accretion rate increases the disc
surface density within a very short time, rendering the disc unstable to
self-gravity and inducing vigorous fragmentation.
