A silicon-containing compound of formula (1), and a curing composition containing a silicon-containing compound of formula (1) wherein Z is hydrogen, a silicon-containing compound of formula (1) wherein Z is C2-C4 alkenyl or alkynyl and a hydrosilylation catalyst. The composition has excellent handling and curing properties and provides a cured product with excellent heat resistance and flexibility. In formula (1), Ra—Rg=C1-C12 saturated aliphatic hydrocarbon group or C6-C12 aromatic hydrocarbon group. Re and Rf do not simultaneously represent C1-C12 saturated aliphatic hydrocarbon group; Y═C2-C4 alkylene; Z=hydrogen or C2-C4 alkenyl or alkynyl; K is 2-7; T is 1-7; P is 0-3; and M and N are numbers selected such that N:M=1:1 to 1:100, that all M's and N's total at least 15, and that the mass average molecular weight of the compound of formula (1) is 3,000 to 1,000,000.