  The mechanism of high-Tc superconductivity is investigated with interests on
the microscopic aspects of the condensation energy. The theoretical analysis is
performed on the basis of the FLEX approximation which is a microscopic
description of the spin-fluctuation-induced-superconductivity. Most of phase
transitions in strongly correlated electron system arise from the correlation
energy which is copmetitive to the kinetic energy. However, we show that the
kinetic energy cooperatively induces the superconductivity in the underdoped
region. This unusual decrease of kinetic energy below T_c is induced by the
feedback effect. The feedback effect induces the magnetic resonance mode as
well as the kink in the electronic dispersion, and alters the properties of
quasi-particles, such as mass renormalization and lifetime. The crossover from
BCS behavior to this unusual behavior occurs for hole dopings. On the other
hand, the decrease of kinetic energy below T_c does not occur in the
electron-doped region. We discuss the relation to the recent obserbation of the
violation of optical sum rule.
