A tag writing device, comprising: a conveying component configured to convey a mount, to which a plurality of wireless tags is attached in a row at about equal intervals, along an array direction of the plurality of wireless tags; an antenna opposed to the mount conveyed by the conveying component, the antenna being provided for wireless communication with a plurality of wireless tags located within a communication range among the plurality of wireless tags; an acquiring component configured to acquire, in a period in which the mount is conveyed by the conveying component from a predetermined start position to a predetermined end position, a plurality of times each time the mount is conveyed by the conveying component by a fixed amount, reception intensities in the antenna of radio signals transmitted from the plurality of wireless tags; a determining component configured to determine, concerning each of the plurality of wireless tags, an intermediate position between the start position and the end position as a position where a sum of reception intensities acquired by the acquiring component while the mount is conveyed from the start position to the intermediate position by the conveying component and a sum of reception intensities acquired by the acquiring component while the mount is conveyed from the intermediate position to the end position by the conveying component are closest; a selecting component configured to select, as a target tag, the wireless tag, the intermediate position of which is determined by the determining component as a position within a prediction range predetermined as a range included in a range between the start position and the end position; and a deciding component configured to decide, based on reception intensity in the antenna of a radio signal transmitted from the target tag selected by the selecting component, a relative positional relation between the wireless tag and the antenna at a time when writing is performed in one of the plurality of wireless tags. a conveying component configured to convey a mount, to which a plurality of wireless tags is attached in a row at about equal intervals, along an array direction of the plurality of wireless tags; an antenna opposed to the mount conveyed by the conveying component, the antenna being provided for wireless communication with a plurality of wireless tags located within a communication range among the plurality of wireless tags; an acquiring component configured to acquire, in a period in which the mount is conveyed by the conveying component from a predetermined start position to a predetermined end position, a plurality of times each time the mount is conveyed by the conveying component by a fixed amount, reception intensities in the antenna of radio signals transmitted from the plurality of wireless tags; a determining component configured to determine, concerning each of the plurality of wireless tags, an intermediate position between the start position and the end position as a position where a sum of reception intensities acquired by the acquiring component while the mount is conveyed from the start position to the intermediate position by the conveying component and a sum of reception intensities acquired by the acquiring component while the mount is conveyed from the intermediate position to the end position by the conveying component are closest; a selecting component configured to select, as a target tag, the wireless tag, the intermediate position of which is determined by the determining component as a position within a prediction range predetermined as a range included in a range between the start position and the end position; and a deciding component configured to decide, based on reception intensity in the antenna of a radio signal transmitted from the target tag selected by the selecting component, a relative positional relation between the wireless tag and the antenna at a time when writing is performed in one of the plurality of wireless tags.