A transmission power control method in a mobile communication system including a user apparatus and a base station is provided. The method enables the base station to control transmission power of a discovery signal of the user apparatus. The method includes estimating a number of the user apparatuses transmitting discovery signals by monitoring the discovery signals transmitted from the user apparatuses, by referring to power class setting information which associates the number of the user apparatuses transmitting the discover signals with a power class used for determining transmission power information of the discovery signal in the user apparatus, determining the power class associated with the number of the estimate user apparatuses, and transmitting the determined power class to the user apparatus.