  The Ish arrangement was introduced by Armstrong to give a new interpretation
of the $q,t$-Catalan numbers of Garsia and Haiman. Armstrong and Rhoades showed
that there are some striking similarities between the Shi arrangement and the
Ish arrangement and posed some problems. One of them is whether the Ish
arrangement is a free arrangement or not. In this paper, we verify that the Ish
arrangement is supersolvable and hence free. Moreover, we give a necessary and
sufficient condition for the deleted Ish arrangement to be free.
