An excavator mounted with tanks having side doors, comprising: a lower driving structure; an upper frame mounted on the lower driving structure to be swiveled; a cabin and an engine room mounted in front and in the rear of the upper frame, respectively; working devices fixed to the upper frame, and composed of a boom an arm, and a bucket driven by hydraulic cylinders (8,9,10) respectively; a counter weight mounted in the rear of the upper frame to keep the balance of the equipment during working; a hydraulic fluid tank and a fuel tank mounted on the upper frame in front of the engine room; characterized by: a side door hingedly fixed to an outer side of the fuel tank to form an outer side surface of the fuel tank; a side door for the engine room hingedly fixed to an outer side of a cowl frame, and extending to a boundary part between the hydraulic fluid tank and the fuel tank to form outer side surfaces of the engine room and the hydraulic fluid tank; and storage parts corresponding to a space between the outer side surface of the fuel tank and an inner side surface of the side door and a space between the outer side surface of the hydraulic fluid tank and the inner side surface of the side door for the engine room, respectively, to keep service components of the excavator in custody. a lower driving structure; an upper frame mounted on the lower driving structure to be swiveled; a cabin and an engine room mounted in front and in the rear of the upper frame, respectively; working devices fixed to the upper frame, and composed of a boom an arm, and a bucket driven by hydraulic cylinders (8,9,10) respectively; a counter weight mounted in the rear of the upper frame to keep the balance of the equipment during working; a hydraulic fluid tank and a fuel tank mounted on the upper frame in front of the engine room; characterized by: a side door hingedly fixed to an outer side of the fuel tank to form an outer side surface of the fuel tank; a side door for the engine room hingedly fixed to an outer side of a cowl frame, and extending to a boundary part between the hydraulic fluid tank and the fuel tank to form outer side surfaces of the engine room and the hydraulic fluid tank; and storage parts corresponding to a space between the outer side surface of the fuel tank and an inner side surface of the side door and a space between the outer side surface of the hydraulic fluid tank and the inner side surface of the side door for the engine room, respectively, to keep service components of the excavator in custody. a side door hingedly fixed to an outer side of the fuel tank to form an outer side surface of the fuel tank; a side door for the engine room hingedly fixed to an outer side of a cowl frame, and extending to a boundary part between the hydraulic fluid tank and the fuel tank to form outer side surfaces of the engine room and the hydraulic fluid tank; and storage parts corresponding to a space between the outer side surface of the fuel tank and an inner side surface of the side door and a space between the outer side surface of the hydraulic fluid tank and the inner side surface of the side door for the engine room, respectively, to keep service components of the excavator in custody.