Provided is a method for efficiently producing a cationically curable condensed silicon compound. There was an unsolved problem that, in a condensation reaction, an oxetanyl group is subjected to ring-opening under an acidic condition, while gelation is easily caused under an alkaline condition. It was found that a silicon compound (C) having an oxetanyl group can be obtained without causing gelation even at a high concentration by the present method including a first step of separately subjecting a silicon compound (A) having four siloxane bond-forming groups and a silicon compound (B) having an oxetanyl group to alcohol exchange reaction with 1-propanol and a second step of subjecting silicon compounds (AP) and (BP) undergone the first step to hydrolytic copolycondensation under an alkaline condition at a specific ratio.