PURPOSE: To provide a farm working machine which can maintain the straight advance performance and the steering wheel operation performance in the excellent manners.

CONSTITUTION: A front weel 1 which can be steered is formed so that the castor angle can be varied, and a castor angle varying driving device 7 is installed, and a working device raising/lowering means and the castor angle varying driving device 7 are interloked so that the csstor angle reduces when a working device 3 installed in free elevation and lowering for the machine body is raised, while the castor angle is increased when the working device 3 is lowered.