A base station according to an embodiment comprises a controller configured to manage a first cell operated using an allocation frequency band allocated by a mobile network operator and a second cell operated using a specific frequency band where occupying a frequency is not allowed by the mobile network operator. The controller performs processes of: transmitting to a mobile station through the first cell, an instruction signal that instructs transmission of a random access preamble through the second cell; and notifying the mobile station, an offset value of uplink transmission timing used in a communication in the second cell, on the basis of the random access preamble received from the mobile station through the second cell.