When a first image forming apparatus 1 cannot perform printing based on a print instruction from a user, the first image forming apparatus 1 (a1) makes a substitution printing request to a second image forming apparatus 2 and (a2) receives a temporary authentication ID from this second image forming apparatus 2 and notifies the user of it if the user is not managed by an authentication server 4 that manages this second image forming apparatus 2. If the user is not managed by the authentication server 4 and not authenticated when receiving the substitution printing request, the second image forming apparatus 2 (b1) requests the authentication server 4 to issue the authentication ID, (b2) receives the authentication ID and notifies the apparatus 1 of it, and (b3) receives a user inputted authentication ID and performs the printing if the inputted authentication ID is identical to the received one.