To obtain a water soluble flux and a metal paste, having a stable viscosity from the start of use and having reduced dispersion of the transfer amount.

In a water soluble flux to be applied through transfer to the parts to be jointed before solder jointing and a metal paste which is the water soluble flux containing metal particles of solder, tin lead, silver, or copper, etc., the initial water content in weight % at the start of use, representing the water content contained in the flux from the first is set to not less than 1%. Thereby the viscosity of the flux can be stabilized from the initial state of the start of use at first, and the time required by a preliminary squeezing action needed hitherto for stabilizing the viscosity can be reduced, and the dispersion of the transfer amount can also be reduced.