  We propose a simple method to approximately evaluate reduced width amplitude
(RWA) of a two-body spinless cluster channel using the norm overlap with the
Brink-Bloch cluster wave function at the channel radius. The applicability of
the present approximation is tested for the $^{16}$O+$\alpha$ channel in
$^{20}$Ne as well as the $\alpha$+$\alpha$ channel in $^8$Be. The approximation
is found to be reasonable to evaluate the RWA for states near the threshold
energy and it is useful to estimate the $\alpha$-decay width of resonance
states. The approximation is also applied to $^9$Li, and the partial decay
width of the $^6$He($0^+_1$)+$t$ channel is discussed.
