An automatic steering system for a working vehicle, for automatic maneuvering of the working vehicle running on a working ground surface along a desired running path, the automatic steering system comprising: a measurement section for acquiring measurements including those of position, velocity, heading direction, acceleration, angular velocity and attitude of the vehicle; and a data processing section; wherein the data processing section comprises: a vehicle related parameter estimator for determining a value of a vehicle related parameter, the vehicle related parameter including a parameter indicating vibration components of the vehicle body; a ground surface related parameter estimator for determining a value of a ground surface related parameter, the ground surface related parameter including a parameter indicating ground surface slipperiness, the parameter indicating ground surface slipperiness including a measured slipperiness parameter based on measurements by the measurement section, and also including an estimated slipperiness parameter, the ground surface related parameter estimator calculating the parameter indicating the vibration components of the vehicle body that are attributable to the ground surface based on the acceleration and the angular velocity of the vehicle and a parameter indicating vibration components of the vehicle body, the ground surface related parameter estimator further calculating the estimated slipperiness parameter by filtering the parameter indicating vibration components of the vehicle that are attributable to a ground surface; and a steering amount value generator for generating a desired steering amount value on the basis of the determined value of the vehicle related parameter and the determined value of the ground surface related parameter; wherein the automatic steering system is adapted for operation (i) in a working mode which is to be adopted when the working vehicle runs on the working ground surface and (ii) in a pre-adjustment mode which is to be adopted in advance to an operation of the automatic steering system in the working mode; wherein, during an operation of the automatic steering system in the pre-adjustment mode, the vehicle related parameter estimator determines the value of the vehicle related parameter on the basis of the measurements acquired by the measurement section; wherein, during an operation of the automatic steering system in the working mode, the vehicle related parameter estimator updates the value of the vehicle related parameter and the ground surface related parameter estimator determines the value of the ground surface related parameter on the basis of the measurements acquired by the measurement section. a measurement section for acquiring measurements including those of position, velocity, heading direction, acceleration, angular velocity and attitude of the vehicle; and a data processing section; wherein the data processing section comprises: a vehicle related parameter estimator for determining a value of a vehicle related parameter, the vehicle related parameter including a parameter indicating vibration components of the vehicle body; a ground surface related parameter estimator for determining a value of a ground surface related parameter, the ground surface related parameter including a parameter indicating ground surface slipperiness, the parameter indicating ground surface slipperiness including a measured slipperiness parameter based on measurements by the measurement section, and also including an estimated slipperiness parameter, the ground surface related parameter estimator calculating the parameter indicating the vibration components of the vehicle body that are attributable to the ground surface based on the acceleration and the angular velocity of the vehicle and a parameter indicating vibration components of the vehicle body, the ground surface related parameter estimator further calculating the estimated slipperiness parameter by filtering the parameter indicating vibration components of the vehicle that are attributable to a ground surface; and a steering amount value generator for generating a desired steering amount value on the basis of the determined value of the vehicle related parameter and the determined value of the ground surface related parameter; wherein the automatic steering system is adapted for operation (i) in a working mode which is to be adopted when the working vehicle runs on the working ground surface and (ii) in a pre-adjustment mode which is to be adopted in advance to an operation of the automatic steering system in the working mode; wherein, during an operation of the automatic steering system in the pre-adjustment mode, the vehicle related parameter estimator determines the value of the vehicle related parameter on the basis of the measurements acquired by the measurement section; wherein, during an operation of the automatic steering system in the working mode, the vehicle related parameter estimator updates the value of the vehicle related parameter and the ground surface related parameter estimator determines the value of the ground surface related parameter on the basis of the measurements acquired by the measurement section.