The present invention provides a silicon nitride-based sintered body having excellent mechanical properties from room temperature to a medium-low temperature range, a low friction coefficient and excellent wear resistance; a raw material powder for the sintered body; a method of producing the raw material powder; and a method of producing the sintered body. The sintered body of the present invention comprises silicon nitride, titanium compounds and boron nitride, or else silicon nitride, a titanium-based nitride and/or carbide, silicon carbide and graphite and/or carbon; and it has a mean particle diameter of 100 nm or less, and a friction coefficient under lubricant-free conditions of 0.3 or less, or else 0.2 or less. The silicon nitride-based composite powder, which is the raw material of the sintered body comprises primary particles of each of silicon nitride and titanium compounds, containing boron or carbon, each having a mean particle diameter of 20 nm or less, or 30 nm or less, and a phase containing an amorphous phase that surrounds the surfaces of the primary particles. Moreover, the method of producing the sintered body comprises pulverizing and mixing a silicon nitride powder, a sintering aid powder, a metallic titanium powder and a boron nitride powder, or else a silicon nitride powder, a sintering aid powder, a metallic titanium powder and a graphite and/or carbon powder, until the mean particle diameters become 20 nm or less, or else 30 nm or less, thus forming secondary