A process for preparing a highly pure silicone ladder polymer of the general formula (1): wherein R1 and R2 represent F, H, a lower alkyl group, an alkenyl group, an aryl group, a lower fluorinated alkyl group, a fluorinated alkenyl group or a fluorinated aryl group; R3, R4, R5 and R6 each represents H, a lower alkyl group or a lower fluorinated alkyl group; and n represents an integer of 5 to 10000, which comprises: (a) a step of obtaining a prepolymer in which at least one organosilane compound is dissolved in an organic solvent and hydrolyzed with ultrapure water; (b) a step of washing the obtained prepolymer with ultrapure water; and, (c) a step of dissolving the washed prepolymer in an organic solvent and heating without a catalyst.