[Problem] To provide a mullite sintered object with excellent chemical resistance, a circuit board using the same, and a probe card on which the circuit board has been applied. [Solution] A mullite sintered object with excellent chemical resistance is obtained, in which the amount of mullite contained is 79.3 to 85.2 wt.%, the amount of aluminum contained is 14.2 to 19.8 wt.% and the amount of MnTiO3 contained is 0.6 to 1.15 wt.%, by adopting a relative density of no less than 96%. By adopting this mullite sintered object as an insulating base (11), a circuit board and a probe card can be obtained that are compact and without irregularities, and in which, during heat load testing, there is little displacement between a measurement terminal provided on the ceramic circuit board for use in the probe card and a measurement pad that is formed on the surface of a Si wafer.
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