  We investigate the dineutron in the $2^+_1$ state of $^6$He via analysis of
its decay mode by using the complex scaling method. In this letter, we propose
the cross section for the resonant state to distinguish the resonant
contributions from the nonresonant ones. As the results, it is found that the
shoulder peak appears in the cross section for the resonant state as a function
of $\varepsilon_{n\text{-}n}$. Furthermore, we show that the $S$ = 0 component
of the cross section, where $S$ is the total spin of the valence two neutrons,
has a peak around the shoulder peak, which comes from the dineutron
configuration in the $2^+_1$ state. Thus we conclude that the shoulder peak is
expected to indicate the existence of the dineutron in the $2^+_1$ state.
