A beam control method for an RFID tag communication system comprising an array antenna in which a directivity direction of a beam of a radio wave is electrically controllable and a RFID tag communication device connected to the array antenna and configured to wirelessly communicate with an RFID tag through the array antenna, wherein the array antenna comprises: exactly four antenna elements (21a - 21d), wherein a second antenna element and a third antenna element of the four antenna elements are arranged spaced apart on a first virtual line, and a first antenna element and a fourth antenna element of the four antenna elements are arranged spaced apart on a second virtual line orthogonal to the first virtual line so as to sandwich the first virtual line, and four variable phase shifters (22 a - 22d), each for variably setting a feeding phase of a respective one of the antenna elements; wherein the method comprises: controlling the feeding phases set by each of the variable phase shifters by controlling voltages supplied to each of the variable phase shifters based on data stored in a table, wherein the table stores, for each of a plurality of different angles θ, the voltages that need to be supplied to each of the variable phase shifters so that the directivity direction of the beam of the radio wave is controlled to be on a plane defined by the first virtual line and an axis orthogonal to both the first virtual line and the second virtual line, and at the angle θ from the axis, and changing the angle θ of the directivity direction of the beam of the radio wave from a first scan angle to a second scan angle by controlling the voltages supplied to each of the variable phase shifters based on the data stored in the table, so that the directivity direction of the beam of the radio wave is changed along the first virtual line. exactly four antenna elements (21a - 21d), wherein a second antenna element and a third antenna element of the four antenna elements are arranged spaced apart on a first virtual line, and a first antenna element and a fourth antenna element of the four antenna elements are arranged spaced apart on a second virtual line orthogonal to the first virtual line so as to sandwich the first virtual line, and four variable phase shifters (22 a - 22d), each for variably setting a feeding phase of a respective one of the antenna elements; wherein the method comprises: controlling the feeding phases set by each of the variable phase shifters by controlling voltages supplied to each of the variable phase shifters based on data stored in a table, wherein the table stores, for each of a plurality of different angles θ, the voltages that need to be supplied to each of the variable phase shifters so that the directivity direction of the beam of the radio wave is controlled to be on a plane defined by the first virtual line and an axis orthogonal to both the first virtual line and the second virtual line, and at the angle θ from the axis, and changing the angle θ of the directivity direction of the beam of the radio wave from a first scan angle to a second scan angle by controlling the voltages supplied to each of the variable phase shifters based on the data stored in the table, so that the directivity direction of the beam of the radio wave is changed along the first virtual line. controlling the feeding phases set by each of the variable phase shifters by controlling voltages supplied to each of the variable phase shifters based on data stored in a table, wherein the table stores, for each of a plurality of different angles θ, the voltages that need to be supplied to each of the variable phase shifters so that the directivity direction of the beam of the radio wave is controlled to be on a plane defined by the first virtual line and an axis orthogonal to both the first virtual line and the second virtual line, and at the angle θ from the axis, and changing the angle θ of the directivity direction of the beam of the radio wave from a first scan angle to a second scan angle by controlling the voltages supplied to each of the variable phase shifters based on the data stored in the table, so that the directivity direction of the beam of the radio wave is changed along the first virtual line.