  We investigate the $e^+p \to \mu^+ X$ event with high transverse momenta
observed at HERA (H1) and show that this event could be interpreted as a
signature of the single production of the scalar top quark in a supersymmetric
model with $R$-parity breaking interactions. The event topology of the H1 event
is rather characteristic and in fact it can be simulated by our specific
scenario if we reasonably choose our model parameters to be ({\romannumeral 1})
$m_{{\widetilde{d}},{\widetilde{b}}, {\widetilde{\nu}}}$
${{\stackrel{>}{\sim}}}$ $1$TeV [$0.8$TeV] for ${\lambda'_{131}}$ $=$ $0.1$
[$0.05$] and ({\romannumeral 2}) $m_{{\widetilde{W}}_{1}}$
${{\stackrel{<}{\sim}}}$ $150$GeV, $100$GeV ${{\stackrel{<}{\sim}}}$
$m_{{\widetilde{t}}_1}$ ${{\stackrel{<}{\sim}}}$ $200$GeV and
${\lambda'_{131}}$ ${{\stackrel{>}{\sim}}}$ $0.05$.
