PURPOSE: To keep depth of seedling transplanting in plural planting rows approximately constant even in the existence of unevenness on the surface of field.

CONSTITUTION: Rise and fall control is carried out in such a way that a seedling transplanting device is maintained at a given height based on the detected result of a first potentiometer PM1 for discovering an vertically rocked amount of a ground float positioning at the center of right and left. A second and a third potentiometers PM2 and PM3 for detecting vertically rocked amounts of ground floats positioning at both the right and left sides, respectively, are installed. When a situation wherein the detected results of the second and the third potentiometers PM2 and PM3 are different from the detected results of the first potentiometer PM1 by ≥ a set value is continued for ≤ a given time, data are changed so as to control the rise and fall of the seedling transplanting device based on the detected results of the second and the third potentiometers PM2 and PM3.