A highly heat-conductive Si-containing material containing a Si phase whose lattice constant at room temperature is controlled at a level of more than 0.54302 nm but 0.54311 nm or less. Firing is conducted using a kiln material containing no B compound. With this highly heat-conductive Si-containing material and the process for production thereof, a reduction in heat conductivity can be prevented and a high heat conductivity can be exhibited stably.