The invention is to provide a polymer, having a high affinity to water, is liquid in spite of being a polymer, simultaneously possesses also a thermal stability and is suitably used as a heating medium or a lubricant. It is the polyacetal (co)polymer, being liquid at room temperature, prepared by copolymerizing (A) 75 to 100 mol % of 1,3-dioxolane and (B) 25 to 0 mol % of trioxane and having the total amount of terminal groups of 15 to 300 mmol/kg and an amount of a hemiformal terminal group of 80 mmol/kg or less.