  We observed the first-ever recorded outburst of PM J03338+3320, the
cataclysmic variable selected by proper-motion survey. The outburst was
composed of a precursor and the main superoutburst. The precursor outburst
occurred at least 5 d before the maximum of the main superoutburst. Despite
this separation, long-period superhumps were continuously seen between the
precursor and main superoutburst. The period of these superhumps is longer than
the orbital period by 6.0(1)% and can be interpreted to reflect the dynamical
precession rate at the 3:1 resonance for a mass ratio of 0.172(4). These
superhumps smoothly evolved into those in the main superoutburst. These
observations provide the clearest evidence that the 3:1 resonance is triggered
by the precursor outburst, even if it is well separated, and the resonance
eventually causes the main superoutburst as predicted by the thermal-tidal
instability model. The presence of superhumps well before the superoutburst
cannot be explained by alternative models (the enhanced mass-transfer model and
the pure thermal instability model) and the present observations give a clear
support to the thermal-tidal instability model.
