A mullite-based body including mullite, alumina, and titanium manganese oxide is disclosed. The mullite-based sintered body includes mullite of 79.3 to 85.2 mass %, alumina of 14.2 to 19.8 mass % and MnTiO3 of 0.6 to 1.1 mass %. The mullite-based sintered body has a relative density of 96% or higher. A circuit board and a probe card are also disclosed. The circuit board includes the mullite-based sintered body. The probe card includes the wiring substrate; a surface wiring layer on a surface of the wiring substrate; and a measuring terminal electrically coupled to the surface wiring layers for measuring the electrical characteristics of an electric circuit.