PURPOSE: To prevent a transmission coupling shaft from rattling, and to ensure safety functions upon overload operation, by providing, between main and auxiliary clutches, an engaging member for disengaging the coupling between both clutches when an overload condition takes place on the auxiliary clutch side.

CONSTITUTION: A transmission coupling shaft 22 is pressed toward a driven shaft 18 by means of a spring 12 which presses a main clutch member 10 against an auxiliary clutch member 11, and therefore, rotation may be transmitted without rattling. Even if any dispersion is appreciated in the lengths of frames 4a, 4b connecting between a main transmission casing 1 and an auxiliary transmission casing 3, the transmission coupling shaft 22 may be surely coupled due to the deflection amount of the spring 12. Further, when an overload condition takes place in a seedling planting device, no rotation of the main clutch member 10 is transmitted to the auxiliary clutch 11 due to the provision of steel balls 30, and the main clutch member 10 slips away overcoming the spring 12 so that a slip takes place between both clutches, thereby no power is transmitted onto the driven shaft 18 side. Accordingly, the seedling planting device may be prevented from failing.