Systems of diagram categories and $K$-theory. I

With any left system of diagram categories or any left pointed dérivateur, a $K$-theory space is associated. This $K$-theory space is shown to be canonically an infinite loop space and to have a lot of common properties with Waldhausen's $K$-theory. A weaker version of additivity is shown. Also, Quillen's $K$-theory of a large class of exact categories including the Abelian categories is proved to be a retract of the $K$-theory of the associated dérivateur.