PURPOSE: To provide a polysiloxane effective as a lubricant for PVC and as an operating oil by hydrolyzing and polymerizing silane, which is obtained by adding itaconate to hydro organo di-chlorosilane.

CONSTITUTION: Itaconate of formula CH2=C(COOR')CH2COOR' (R' means a C1-12 saturated monovalent hydrocarbon group, C2-12 alcoxyalkyl) is added to hydro organo di-chlorosilane of a formula HSiRaCl3-a (R means C1-20 alkyl, vinyl, phenyl 3,3,3-trifuloropropyl; (a) means 1, 2) so as to obtain a formula ZSiRaCl3-a>>Z, and silane of -CH2CH(COOR') CH2COOR', -C (CH2) (COOR') CH2COOR'] is hydrolyzed and polymerized so as to obtain di-organo polysiloxane having one or more unit of a formula ZRaSiO(3-a)/2 as di-organo polysiloxane selected from a formula I (Z' means R, Z; (r) means 0-500; (s) means 0-50) and a formula II ((u) means 1-20; (t) means 0-20; (t)+(u) means 3 or more).