part 46b of the guide groove with the compression spring 44 compressed as shown in fig8 and 6 in this state the user pushes inwardly against the front end 36 of the inner casing 30 with a suitable force whereupon the compression spring 44 is compressed to cause the free end 45f of the stopping hook 45 to move away from the insertion catch part 46b into the projection guide part 46d as shown in fig1 so that the resilient recuperative force of the compression spring 44 causes the catch member 41 to project out of the catch housing 42 as shown in fig7 when the projecting motion of the catch member 41 is completed the free end 45f of the stopping hook 45 is positioned in the projection catch part 46a as shown in fig9 meanwhile as the catch member 41 is retracted from the catch housing 42 the resilient hook 43 of the catch member 41 is released from the hook 27 because the ceiling 42g moves out of the overlying relationship with the hook 43 thus releasing the inner casing 30 from the outer casing 20 namely upon releasing the resilient hook 43 the inner casing 30 starts the retracting motion by means of the resiliently recuperative force of the plate spring 53 in this case the gear wheel 62 mounted on the inner casing 30 rotates engaged with the rack 61 whereupon the impeller 65 attached to the rotating shaft 63 rotates retarded by the oil of the oil reservoir 64 so as to.