A tractor including a ground work implement liftably mounted to a traveling machine body which is caused to travel with a traveling apparatus, the tractor comprising: a draft load sensor for detecting a draft load value during a work by the ground work implement; draft controlling means for executing lifting up/down control of the ground work implement based on the draft load value detected by the draft load sensor; slip ratio acquiring means for acquiring a slip ratio of the traveling apparatus relative to the ground surface; and control information changing means; and a storage; wherein the storage is configured to store a plurality of control information, characterized in that each of the control information has a data structure displaying relationship between the draft load value and a work implement level as linear information, wherein during a draft control by the draft controlling means, the draft controlling means are configured to obtain a target work implement level, that is a level of the work implement relative to the machine body, by referring to the control information, with the draft load value detected by the draft load sensor, the control information stored in the storage; and the lifting up/down control of the ground work implement is executed so as to achieve agreement between said target work implement level and a detected work implement level that is a level of the work implement relative to the machine body detected by a level sensor; when the slip ratio acquired by the slip ratio acquiring means exceeds a threshold value, said control information changing means are configured to newly set the control information which renders higher the target work implement level relative to a same draft load value, in place of the control information which has been set already during the draft control, or said control information changing means is configured to enhance control sensitivity of the control information which has already been set during the draft control. a draft load sensor for detecting a draft load value during a work by the ground work implement; draft controlling means for executing lifting up/down control of the ground work implement based on the draft load value detected by the draft load sensor; slip ratio acquiring means for acquiring a slip ratio of the traveling apparatus relative to the ground surface; and control information changing means; and a storage; wherein the storage is configured to store a plurality of control information, characterized in that each of the control information has a data structure displaying relationship between the draft load value and a work implement level as linear information, wherein during a draft control by the draft controlling means, the draft controlling means are configured to obtain a target work implement level, that is a level of the work implement relative to the machine body, by referring to the control information, with the draft load value detected by the draft load sensor, the control information stored in the storage; and the lifting up/down control of the ground work implement is executed so as to achieve agreement between said target work implement level and a detected work implement level that is a level of the work implement relative to the machine body detected by a level sensor; when the slip ratio acquired by the slip ratio acquiring means exceeds a threshold value, said control information changing means are configured to newly set the control information which renders higher the target work implement level relative to a same draft load value, in place of the control information which has been set already during the draft control, or said control information changing means is configured to enhance control sensitivity of the control information which has already been set during the draft control.