  We study the low-energy effective theory of N=2 supersymmetric Yang-Mills
theory with the exceptional gauge group $G_{2}$. We obtain the Picard-Fuchs
equations for the $G_{2}$ spectral curve and compute multi-instanton
contribution to the prepotential. We find that the spectral curve is consistent
with the microscopic supersymmetric instanton calculus. It is also shown that
$G_{2}$ hyperelliptic curve does not reproduce the microscopic result.
