sensor once the scavenge time has been set in step s2 control proceeds to step s3 scavenge device to execute actual processing for scavenging the fuel vapor inside the piping more specifically the purge control valve 23 is opened the one port of the switching valve 26 is closed the other port is opened and the electric pump 28 is driven by applying a constant voltage thereto at this time as shown in fig3 due to operation of the electric pump 28 air introduced via the air filter 30 and the air introduction passage 29 passes via the first passage 25 through the canister 21 and is discharged into the intake air passage 3 via the purge passage 22 furthermore a part of the air passes from the switching valve 26 via the air discharge passage 31 and the air filter 30 and is discharged into the atmosphere as a result the residual pressure negative pressure and residual gas inside the purge passage 22 is scavenged and thus eliminated then in step s4 a basic value dlslst of the leak diagnosis judgement level is determined more specifically the purge control valve 23 is closed the one port of the switching valve 26 is closed the other port is opened and the electric pump 28 is driven and this condition is maintained for a predetermined time at this time as shown in fig4 due to operation of the electric pump 28 air introduced via the air filter.