  We consider irrational fixed points of the Minkowski question mark function
$? (x)$, that is irrational solutions of the equation $? (x)=x$. It is easy to
see that there exist at least two such points. Although it is not known if
there are other fixed points, we prove that the smallest and the greatest fixed
points have irrationality measure exponent equal to 2. We give more precise
results about the approximation properties of these fixed points. Moreover, in
Appendix we introduce a condition from which it follows that there are only two
irrational fixed points.
