  Why the charged lepton mass formula m_e +m_\mu +m_\tau = {2/3}
(\sqrt{m_e}+\sqrt{m_\mu} +\sqrt{m_\tau})^2 is mysterious is reviewed, and
guiding principles to solve the mystery are presented. According to the
principles, an example of such a mass generation mechanism is proposed, where
the origin of the mass spectrum is attributed not to the structure of the
Yukawa coupling constants, but to a structure of vacuum expectation values of
flavor-triplet scalars under Z_4 \times S_3 symmetries.
