A conventional silsesquioxane derivative has the problems that the functional groups are restricted and the chemical structure is not readily controlled and that it is expensive. The present inventors have developed a process for producing a silsesquioxane derivative at a high yield by a simple process in order to solve such problems. The novel silsesquioxane derivative according to the present invention is controlled in a structure thereof and has a functional group, which is excellent in reactivity with a target compound, to be modified. The present invention relates to a production process for a silsesquioxane derivative represented by Formula (2), characterized by using a silicon compound represented by Formula (1). In Formula (1) and Formula (2), R is a group selected from hydrogen, alkyl, aryl and arylalkyl; M is a monovalent alkaline metal atom; at least one of Y is a group represented by Formula (3), and the remainder of Y is hydrogen; R1 and R2 in Formula (3) represent the same group as defined for R; and Z is a functional group or a group having a functional group