To provide a magnetic stripe writing device for suppressing the speed variation of a medium at the time of writing data in a magnetic stripe installed in a medium.

This magnetic stripe writing device is provided with a magnetic head 23 for moving a horizontal medium 12 equipped with a magnetic stripe 33 by a conveyance means 20 which conveys the medium 12 in a state that the medium is opened, and for writing data in the magnetic stripe 33, a medium thickness detecting part 25 for detecting the thickness of the medium 12 at a top end side 30 and a control part 41 for detecting the thickness of the medium 12 by the medium thickness detecting part 25, and conveying the medium 12 to the magnetic head 23 from the thin side, and writing the data in the magnetic stripe 33.