PROBLEM TO BE SOLVED: To provide a technique capable of easily determining a dividing position of a field. SOLUTION: A field marking device 100 for setting a line for dividing a field, wherein a width of a work target area of a working machine for performing agricultural work while moving is Lu, and division after separating by the line. With the width of the field being L, L = (Lu × 2n) is satisfied, and the direction in which the working machine is carried into the field and the direction in which the working machine is carried out from the field are different by 180 °. The writing stroke line calculation unit 103 that calculates the position of the line under the condition is provided. [Selection diagram] Figure 1【課題】圃場の分割位置を簡便に決めることができる技術の提供をする。【解決手段】圃場を分割するラインの設定を行う圃場分筆装置１００であって、異動しながら農作業を行う作業機の被作業対象となる領域の幅をＬu、前記ラインによって分離された後の分割圃場の幅をＬとして、Ｌ＝（Ｌu×２ｎ）を満たし、且つ、前記圃場への前記作業機の搬入の方向と前記圃場からの前記作業機の搬出の方向とが１８０°異なる方向となる条件で前記ラインの位置を算出する分筆ライン算出部１０３を備える。【選択図】図１
