To enable the oil pressure sensitivity of a float to be controlled depending on the change of the revolution of an engine and the running speed of a machine body, and improve the followability to the surface of a field.

A riding-type rice transplanter 10 has the following construction: the posture of a transplanting part 24 vertically moved and supported by an oil hydraulic cylinder 30 is perceived by the buoyancy acting upon a float 27; and an oil pressure controlling valve 32 for controlling the cylinder 30 is linked to the float 27 via a cooperation mechanism 35 to control the valve 32 by the change of the posture of the float 27. The cooperation mechanism 35 has an oil pressure sensitivity-controlling lever 31 which can control the oil pressure sensitivity of the float 27. The lever 31 is linked to an engine throttle lever 20 and a main speed-changing lever 23 so that the oil pressure sensitivity of the float 27 can be changed according to the change of the revolution of the engine and the running speed of the machine body, and the followability of the float 27 to the surface of an field is not damaged even on the speeding up of working.