PURPOSE: To automatically guide a low speed mobile car to a side line track, to improve safety by way of preventing collision and to automatically stop the mobile car by connecting to a main line track, the side line track free to be switched provided with a collision preventive function on which the mobile car for towing a hydroski.

CONSTITUTION: To drive a mobile car 1 for towing a hydroskier 10, a main line track 3a and a side line track 3b are set in parallel on a water surface 16, and simultaneously, a branching device 60 for switching respective tracks 3a, 3b is installed. Additionally, a first opening and closing contact of a mobile car 1 is connected to each contact point to close at the time when the mobile car 1 turn at non-low speed. Furthermore, a second opening and closing contact of the mobile car 1 is connected to each contact point to close at the time when the mobile car 1 runs at low speed. Thereafter, a contact body C31 connected to a circuit S1 to switch the branching device 60 to the side of the main line track 3a is arranged on the main line track 3a to make contact with the first opening and closing contact. In the meantime, a contact body C32 connected to a circuit S2 to switch the branching device 60 to the side of the side line track 3b is arranged on the main line track 3a to make contact with the second opening and closing contact.