  We clarify some ambiguous points in a derivation of duality via brane
exchange using M-theory language, and propose a ``proof'' of duality in MQCD.
Actually, duality in MQCD is rather trivial and does not need a complicated
proof.
  The problem is how to interpret it in field theory language. We examine BPS
states in N=2 theory and find the particle correspondence under duality. In the
process, we also find some exotic particles in N=2 MQCD, and we observe an
interesting phenomenon in type IIA string theory, namely, that fundamental
strings are converted into D2-branes via the exchange of two NS5-branes. We
also discuss how we should understand Seiberg's N=1 duality from exact duality
in MQCD.
