to enter the guide recessed parts 31 j 31 k of the spout 31 more specifically the fractional parts 34 c 34 d that are being pushed in from a peripheral surface part of the guide recessed parts 31 j 31 k of the spout 31 located just thereabove are first deformed inside as a whole by permitting tapered faces thereof to abut on against some own elastic force because of its being made of synthetic resin the peripheral surface and are then elastically restored to an original state by permitting them to enter the guide recessed part in this restored state the fractional parts 34 c 34 d are engaged with the guide recessed parts 31 j 31 k in the operation button 34 assembled finally the pair of the leaf springs abut on the upper outer peripheral surface 31 g of the spout 31 such that they are displaced outside a little and a single fractional part 34 b for driving needle valve abuts on the receiving part 2 e of the needle valve 2 the pair of the leaf springs 34 a are formed symmetrically with respect to a central line that passes through a fractional part 34 b of the top surface of the operation button 34 the operation button 34 is supported by the spout 31 with good balance at the pair of the leaf springs 34 a and at a single fractional part 34 b and is positioned peripherally.