A slip supply system for supplying slip to one or more casting molds includes a slip supply pipe 2 to which the base ends of pouring hoses 3 each having a pouring valve 4 are connected. The ends of the pouring hoses 3 are detachably connected to casting molds 1. A slip discharge pipe 20 having communicating holes 22 is laid in the vicinity of the slip supply pipe 2. The ends of the pouring hoses 3 are removed from the casting molds 1 and connected to the communicating ports 22 before supplying slip to the casting molds 1. Slip is fed in this condition, so that the solid matters attached to the interior of the slip supply pipe 2 and the interiors of the pouring hoses 3 are washed away to the slip discharge pipe 20. Thereafter, the ends of the pouring hoses 3 are connected to the casting molds 1 to feed slip to the casting molds 1.