Flame-retardant polyorganosiloxane composition, flame-retardant cured product, optical member, light source lens or cover, and molding method

A flame-retardant polyorganosiloxane composition comprising:

• (A) a straight-chain polyorganosiloxane having two or more alkenyl groups bonded to silicon atoms on average in one molecule and having a viscosity at 25° C. of 10,000 to 1,000,000 mPa·s;
• (B) 30 to 80 mass % of a polyorganosiloxane relative to a total of the (A) component and this component, the polyorganosiloxane having a resin structure which includes a tetrafunctional siloxane unit represented by a formula:
• SiO4/2, has one or more substituted or unsubstituted alkyl groups bonded to silicon atoms in one molecule and “0” (zero) or more alkoxy groups bonded to silicon atoms in one molecule, and has 1.5 or more alkenyl groups bonded to silicon atoms on average in one molecule and in which a molar ratio (a number of moles of alkoxy groups/a number of moles of substituted or unsubstituted alkyl groups) of the alkoxy groups to the substituted or unsubstituted alkyl groups is 0.030 or less;
• (C) an amount of a polyorganohydrogensiloxane which has hydrogen atoms bonded to silicon atoms, whose average degree of polymerization is 10 or more, and in which a content of the hydrogen atoms is 5.0 mmol/g or more, in which the hydrogen atoms in the component are 1.0 to 3.0 mol relative to a total of 1 mol of the alkenyl groups in the (A) component and the alkenyl groups in the (B) component;
• and
• (D) a catalyst amount of a hydrosilylation reaction catalyst.