An expansion tank device 14 comprises a tank installation base 16 having a cooling liquid channel 17 and an expansion tank 18 provided on the upper surface of the installation base 16. The base 16 has a communication hole 19 for holding space above the upper surface thereof in communication with the cooling liquid channel 17. The expansion tank 18 has a tank main body 21 including an upwardly bulging portion 22 having an opening at its lower end, and a bottom plate 23 joined to the lower end of the tank main body 21 for closing the lower-end opening of the bulging portion 22 and joined to the upper surface of the tank installation base 16. The bottom plate 23 is provided at a portion thereof corresponding to the communication hole 19 with a through hole 25 communicating with the communication hole 19. The bottom plate 23 is provided with a baffle plate 26 formed along the entire circumference of the inner peripheral edge thereof defining the through hole 25 and slanting upwardly toward the center of the through hole 25. The expansion tank device 14 removes the air in the cooling liquid from the circulating system, precludes the system from breaking and prevents the decrease in the amount of cooling liquid within the circulating system.