title: Conditions for Gravitational Instability in Protoplanetary Disks authors: Shigeo Kimura & Toru Tsuribe abstract: Gravitational instability is one of considerable mechanisms to explain the formation of giant planets. We study the gravitational stability in the protoplanetary disks around a protostar. The temperature and Toomre's Q-value are calculated by assuming local equilibrium between viscous heating and radiative cooling. Then, we derive the critical surface density that is necessary for a disk to become gravitationally unstable as a function of radius. At the radius where ices form, the value of critical surface density changes discontinuously by one order of magnitude. By comparing a given surface density profile to the critical surface density, one can discuss the gravitational instability in protoplanetary disks. As an example, we discuss the gravitational instability by using a semi-analytic model for protoplanetary disks in the framework of the steady state accretion disk, which is realized after the viscous evolution. As a result, it is found that the disks tend to become gravitationally unstable in the farther region than the snow line because ices enable the disks to become low temperature.