A tilt detection system includes an optical disk having a series of data recorded therein, the series of data having a pulse width smaller than the minimum pulse width of data recorded in an ordinary optical disk; a reproducing part reproducing a signal from the optical disk by emitting a light beam onto the optical disk; and a tilt detecting part detecting a tilt of the light beam with respect to the optical disk based on a reproduced RF signal provided by the reproducing-part when the series of data having the smaller pulse width is reproduced.