  For a metric graph $G=(V,E)$ and $R\subset V$, the internal Steiner minimum
tree problem asks for a minimum weight Steiner tree spanning $R$ such that
every vertex in $R$ is not a leaf. This note shows a simple polynomial-time
$2\rho$-approximation algorithm, in which $\rho$ is the approximation ratio for
the Steiner minimum tree problem. The result improves the previous best
approximation ratio $2\rho+1$ for the problem. The ratio is not currently best
but the algorithm is very simple.
