  When an exotic hadron locates near the threshold with the channel couplings,
the internal structure of the exotic hadron is related to the scattering
length. To incorporate the threshold effect, the Flatt\'{e} amplitude has been
often used to determine the scattering length. It is however known that an
additional constraint is imposed on the Flatte amplitude near the threshold. We
discuss this problem by using the effective field theory for the
coupled-channel scattering.
