To uniformly surface-treat dendritic copper powder and to obtain a copper paste stabilized in initial resistance value at the time of charging a mixture of the dendritic copper powder and surface-treating agent into a rotary-vessel mixer by specifying the charge of the dendritic copper powder with respect to the volume of the mixer.

A dendritic copper powder and a surface-treating agent are mixed and charged into a rotary-vessel mixer to produce the the copper powder material for a copper paste. In this case, the charge of the dendritic copper powder is controlled to 5-35% of the volume of the mixer or preferably to 10-30%. Consequently, the dendritic copper powder and surface-treating agent are efficiently mixed, and the dendritic copper powder is uniformly surface- treated. An electrolytically produced dendritic copper powder, etc., are used in this case. The average grain diameter of the copper powder is appropriately controlled to 2-20μm and the tap density to ≤3.3g/cm3. A mixture of 0.05-5 pts.wt. of the saturated or unsaturated higher fatty acid or its salt as a surface- treating agent and 0.05 pts.wt. of the copper powder is used.