The present invention relates to a reservoir tank for an automobile, which is provided with a height-adjustable sub filler neck so as to be retracted when not in use and extracted when filling coolant, whereby the reservoir tank can be disposed low in the automobile. The reservoir tank forming a cooling module together with a condenser, a radiator and a fan and shroud assembly is disposed at a carrier, and includes a filler neck for filling coolant and a cap for closing the filler neck, and is communicated with a radiator so as to control an amount of the coolant in the radiator, wherein the reservoir tank is integrally formed with a shroud of the fan and the shroud assembly, and the filler neck is formed with a sub filler neck which is retracted into and extracted from the filler neck.