A base station that communicates with a mobile station that intermittently performs a receiving operation during a cyclic reception period includes: an assignor that assigns a preamble to the mobile station; a start request signal generator that generates a start request signal containing the preamble assigned to the mobile station and requesting the start of a synchronization process between the mobile station and the base station; and a beam controller that transmits start request signals in a plurality of directions within the reception period while switching directions of directional beams for transmitting start request signals, and determines the direction of the directional beam for communicating with the mobile station, in accordance with the direction of the directional beam at the time of reception of the preamble transmitted from the mobile station having received the start request signal.