PROBLEM: To provide a cover body (18) at a space X formed between a motor housing (14) and a sprocket (15) which are the components of a drive sprocket (10) of a base carrier (2) in a hydraulic shovel, so as to prevent foreign substances such as sand from entering into the space, and to be installed to the motor housing side (14) without forming a bolt hole. SOLUTION: A cover body (18) is formed to be a split-body divided to an upper half (19) and a lower half (20), and to be connected to each other to form a ring shape by connecting a connect bolt (23) formed at an adjacent part of the cover body (18). The cover body (18) is installed to the space X by inserting an adjust bolt (21) into a ring shaped main body part (19a and 20a) till abut the motor housing (14) tightly. An outlet (26) is formed as a notch on a cover body (19d and 20d) to discharge the foreign substances entering into the space X between the sprocket housing (15) and a tip end of the cover part.