Proposed is an operation method of a first device (100) in a wireless communication system. The method may comprise the steps of: receiving a configured grant from a base station (300); obtaining a first HARQ process identifier (ID) related to a first period of the configured grant; transmitting, to a second device (200), a first PSSCH, on the basis of a first resource included in the first period; obtaining a second HARQ process ID related to a second period of the configured grant; and transmitting, to the second device (200), a second PSSCH, on the basis of a second resource included in the second period.