PROBLEM TO BE SOLVED: To provide a deterioration determination method for a teat cup liner and a teat cup for a milking machine, allowing accurate and easier detection of deterioration of the teat cup liner.SOLUTION: A teat cup liner 30 is equipped with, on the outer surface of the teat cup liner 30, a mark part 35 provided in a portion having prescribed positional relation with a reference position part 25 present in a teat cup shell 20 in a state that a head part 31 is locked to one open end part 21 and that an engagement part 33 is not engaged with the other open end part 22. When determining deterioration of the teat cup liner 30, it is determined the replacement time of the teat cup liner 30 has come when the mark part 35 of the teat cup liner 30 is positioned on the other open end part 22 side to the reference position part 25 of the teat cup shell 20 over a prescribed reference.
