Recent advances of our study of magnetic properties of a series of molecular conductors, Cu salts of DCNQIs, are presented. First, we have found weak ferromagnetism in the antiferromagnetically ordered state of Cu spins in the insulating phase. Second, the metal-insulator (M-I) transitions of these systems have been characterized by the abrupt change of Cu 3d spins from the itinerant state to the completely localized state, even in the metal-insulator-metal (M-I-M) reentrant system, (DMe-DCNQI-α,α'-d2)2Cu. Finally, (DI-DCNQI)2Cu is found to exhibit enhanced and temperature dependent susceptibility, suggesting the effect of spin fluctuations of itinerant electrons in a Cu 3d narrow band.