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 ( \dot { P _ { sh } } / P _ { sh } ) was positive through the superoutburst .
The superhump period excess ( \varepsilon \equiv P _ { sh } / P _ { orb } -1 ) 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 m _ { V } \sim 13 and two types of superoutbursts : one has a short duration ( around 10 days ) and faint maximum magnitude ( m _ { V } \sim 12.5 ) , the other has a long duration ( around 20 days ) and bright maximum ( m _ { V } \sim 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 .