The paper deals with the energy and the optical spectra of Frenkel biexciton (bound two-exciton state, excitonic molecule) in molecular crystal. A very simple system of equations describing the biexciton states is obtained. The number of equations in this system is equal to the number of the nonzero matrix elements of dynamical exciton interaction. In contrast to the other papers, the kinematical. interaction of excitons is included into the Green’s functions, which are equivalent to the Green’s functions for one exciton states in a crystal with a vacancy. The obtained system of equations is solved for one-, two-, and three-dimensional crystals taking into account only the isotropic nearest-neighbor interaction. It is shown that biexciton has anisotropic character in two-and three-dimensional crystals for certain quasimomentum directions, i.e., it can be stretched along one axis or plane. In the isotropic case the wave function of a biexciton is analogous to the s or d function of the hydrogen atom. This system of equations can be easily solved for more complex models as well. The transition rates from the ground state to the biexciton band (two-photon absorption) and from the one-exciton band (one-photon absorption) to the same states is investigated, too. It is shown, that transition is allowed only into the s biexciton state, and it has a resonance character analogous to the Rashba effect for the localized one-exciton state in impurity crystal.