The invention solves the problem of overloading intermediate routers with state information as the number of multicast groups increases to millions of groups. Generation identifiers, which are updated each time a synchronization occurs, are assigned to objects in a store. When the flaw detecting signal is outputted in a time period in which the jump controller controls the first track jump corresponding to the still reproduction mode, the jump controller sets a timing for stating the second track jump to a timing different from a timing for starting the first track jump. Adjacent protrusions form mounting grooves for retaining the WDMs therein.