A ceramic substrate according to one aspect of the invention comprises a main surface wherein a diameter of a first imaginary circle inscribed in a circumference of the main surface is represented by C (m), a thickness of the ceramic substrate is represented by h (m), a Poisson ratio of the ceramic substrate is represented by ν, a density of the ceramic substrate is represented by ρ(kg/m3), a Young's modulus of the ceramic substrate is represented by E(Pa), a deflection of the ceramic substrate is represented by X (m), the deflection being measured in a condition that the ceramic substrate is supported in a region between the first imaginary circle and a second imaginary circle which is a concentric circle of the first imaginary circle and has a diameter of C-0.01 (m), wherein X≦1.5×(C4/h2)×A and A={3×9.807×ρ×(1−ν2)×(5+ν)}/{28×(1+ν)×E} are satisfied.