PROBLEM TO BE SOLVED: To provide a pretreatment material used for obtaining impressions with a dental alginate impression material, which enables obtaining precise impressions without the need for adjusting the timing of hardening of each impression material when a combined impression is obtained using an alginate impression material.SOLUTION: A pretreatment material for obtaining alginate impressions contains (A)alginate, (B)surfactant, and (C)water and is characterized by the viscosity ranging from 5 to 80 Pa s at 23°C.