  We investigated the optical properties of (Ca,Sr)RuO_3 films on the
borderline of a metal-insulator (M-I) transition. Our results show all of the
predicted characteristics for a metallic Mott-Hubbard system, including (i) a
mass enhancement in dc-limit, (ii) an U/2 excitation, and (iii) an U
excitation. Also, a self-consistency is found within the
Gutzwiller-Brinkman-Rice picture for the Mott transition. Our finding displays
that electron correlation should be important even in 4d materials.
