  An approximate rotational symmetry of a heavy-light meson is viewed from a
string picture. Using a simple string configuration, we derive a formula,
$(M-m_c)^2=\pi\sigma L$, whose coefficient of the r.h.s. is just 1/2 of that of
a light meson with two light quarks. A numerical plot is obtained for $D$
mesons of experimental data as well as several theoretical models, which shows
good agreement with this formula.
