Abstract: The present invention is directed to a novel functional polyorganosiloxane having on average two or more vinyl groups in one molecule thereof. The polyorganosiloxane is represented by the following formula (I): (R23SiO1/2)a(Ph2SiO2/2)b(R1SiO3/2)c(PhSiO3/2)d(R1R2SiO2/2)e??(I) (wherein “a” to “e” represent compositional proportions by mole, satisfying the conditions: 0.15?a?0.4, 0.1?b?0.2, 0.15?c?0.4, 0.2?d?0.4, 0?e?0.2, and “a+b+c+d+e”=1; R1 represents a vinyl group; R2 represents a methyl group or a phenyl group; and Ph denotes a phenyl group) and is produced through polycondensation of an alkoxysilane mixture in the presence of an active solvent. The present invention also relates to a curable resin composition containing the polyorganosiloxane, which composition is suitably employed for encapsulating optical devices such as LEDs, photo-sensors, and lasers as well as optical materials.