This essay tries to justify the claim that the problem of madness is crucial for existentialist philosophy of J.-P. Sartre. Two sorts of arguments are given. First, Sartre worked out a theory of emotions propounding that some sane feelings may be deeply inimical to the world we live in. The mental state of a madman need not be seen as a sign of of his insanity but, instead, may be an indication of his deep but disapproving insight into the nature of being. Secondly, the madman is in most cases an alienated and lone individual who is doomed to struggle with society as well as with his own 'etre-en-soi'. In this respect, he paralells an existentialist who is capable of rising above his indifference and bad faith to take full responsibility for whatever he/she will or will not do.