Provided is an organic silicon oxide fine particle capable of satisfying an expected dielectric constant and mechanical strength and having excellent chemical stability for obtaining a high-performance porous insulating film. More specifically, provided is an organic silicon oxide fine particle comprising a core comprising an inorganic silicon oxide or a first organic silicon oxide containing an organic group having a carbon atom directly attached to a silicon atom and, and a shell on or above an outer circumference of the core, the shell comprising a second organic silicon oxide different from the first organic silicon oxide which the second organic silicon has been formed by hydrolysis and condensation, in the presence of a basic catalyst, of a shell-forming component comprising an organic-group-containing hydrolyzable silane containing an organic group having a carbon atom directly attached to a silicon atom or a mixture of the organic-group-containing hydrolyzable silane and an organic-group-free hydrolyzable silane not having the organic group, wherein a ratio [C]/[Si] is 0 or greater but less than 1 in the core and 1 or greater 1 in the shell wherein [C] represents the number of all the carbon atoms and [Si] represents the number of all the silicon atoms.