To diagnose the abnormality of an evaporating fuel treatment device in a short time.

The evaporating fuel treatment device 12 to be subjected to malfunction diagnosis by a abnormality diagnosis device 25 comprises a bypass on-off valve (a bypass VSV 18) provided in a bypass 14 between a canister 15 and a fuel tank 11, a purge on-off valve (a purge VSV 19) provided in a bypass 16 between the canister 15 and an internal combustion engine 31, and an atmospheric air introducing on-off valve (a CCV 20) provided in an atmospheric air introduction passage 17. The malfunction diagnosis device 25 comprises a pressure sensor 21, an electronic control unit (an ECU 22) and an external diagnosis device 23. The pressure sensor 21 detects an inner pressure (a tank inner pressure) of the fuel tank 11. The ECU 22 and the external diagnosis device 23 output command signals for opening both the bypass VSV 18 and the purge VSV 19 and closing the CCV 20 and diagnose the conditions of both VSVs 18, 19 in accordance with the degree of a change in the inner pressure of the tank after the output.