  We carried out time-resolved photometric observations of an SU UMa type dwarf
nova, BC UMa, during its superoutburst in February 2003. We detect early
superhumps (or outburst orbital humps) during the first two days of the
outburst. The period of early superhumps is 0.06258(13) d and the amplitude is
0.04 mag. After the early superhump phase, common superhumps with an amplitude
of 0.3 mag and a period of 0.064466(16) d developed. The change rate of
superhump periods was positive through the superoutburst. The superhump period
excess is 3% and we derive a mass ratio of 0.13. This is twice as large as that
of WZ Sge, suggesting that the mechanism of early superhumps in BC UMa is not
the 2:1 resonance which was proposed in WZ Sge.
  We have modelled early superhump light curves including irradiation effects
of the accretion disk and secondary star by the white dwarf and accretion disk.
The observed early superhumps can be reproduced when two-armed spirals appear
on the accretion disk.
  We have found the long term data taken from AAVSO, VSOLJ and VSNET shows BC
UMa has normal outbursts with a maximum magnitude of V~13 and two types of
superoutbursts: one has a short duration (around 10 days) and faint maximum
magnitude (V~12.5), the other has a long duration (around 20 days) and bright
maximum (V~11-11.5). BC UMa is the first example of dwarf novae showing the two
types of superoutbursts. The supercycle of BC UMa is between 600 days and 1000
days. This is shorter than WZ Sge type dwarf novae and longer than normal SU
UMa type stars. This phenomenon suggests that BC UMa is an intermediate dwarf
nova between WZ Sge and SU UMa.
