PURPOSE:To reduce sinking and floating to soil face regardless of weight change of a work machine and carry out stable pitching control, by providing correcting device for correcting detection sensitivity by float sensor according to the weight of the work machine. CONSTITUTION:A pitching correction device 5 is provided on an upper link 11 and pressure of a spring 35 is applied between a piston 34 and pivotal axis 17 of the pitching correction device 5 and the hydraulic circuit of an elevation cylinder 20 is branched off and connected to the device 5 against the spring 35. In change-over control by elevation control valve, oil pressure works also to the pitching correction device 5 with oil pressure to the elevation cylinder 20 and the tension of the spring 35 is changed. When the weight of seedling planting apparatus is great, the tension of spring 35 becomes strong and detection sensitivity by sensor 4 is sensitized and changeover to ascending position of the elevation controlling valve is sensitized. When the weight of seedling planting apparatus is small, the tension is also small and detection sensitivity is insensitized.
