A polyorganosiloxane comprising an M unit (R1R2R3SiO1/2) at a content of 10% by mol or more relative to the total of silicon and a T unit (R6SiO3/2) at a content of 80% by mol or less relative to the total of silicon, the polyorganosiloxane having an alkoxy group bound to silicon, and a reactive functional group bound to silicon, other than an alkoxy group, wherein the polyorganosiloxane has the alkoxy group bound to silicon at a content of 0.07 to 4% by weight based on the total weight of the polyorganosiloxane and has 3 to 12 of the reactive functional groups bound to silicon on a number basis per a molecular weight of 1000 of the polyorganosiloxane, and a weight loss of the polyorganosiloxane in heating at 110° C. under a reduced pressure of 0.15 torr for 2 hours is 5% by weight or less, provided that R1, R2, R3, and R6 are each independently an organic functional group other than an alkoxy group and a hydroxyl group, or a hydrogen atom, and are optionally the same or different from one another.