PURPOSE: To perform a high-precsie detection of a falling angle of a culm, by a method wherein a non-contact type distance sensor detects a reaped and an unreaping ground, and the detecting signal is processed by a controller.

CONSTITUTION: A harvester works such that culms 13, planted in a filed, are pushed aside by a pushing-aside device 6 of a preprocessing part 3 for reaping, they are raised up by a raising-up device 7, they are reaped by a reaper, and they are threshed while they are being conveyed backward. A non-contact type distance sensor 15 is mounted to the top part of the raising-up device 7 in a combine C, and it detects a distance to a detecting point at a detecting region having a given width ranging between a reaped and an unreaping ground in an advancing direction and ahead of a machine to input the dtetcting signal to a controller. Based on the unevenness of the surface of the reaped ground, a reaping height target surface S is set, the height of the preprocessing part 3 for reaping is regulated to the target surface S, the falling condition of the cilms 13 is detected, and a speed of the combine C is regulated to the falling condition.