An electrical testing substrate unit includes a multi-layer ceramic substrate formed of mullite and a borosilicate glass as predominant ceramic components. In the multi-layer ceramic substrate, the borosilicate glass contains an alkali metal oxide in an amount of 0.5 to 1.5 mass %. The multi-layer ceramic substrate has a mean coefficient of linear thermal expansion having a value of 3.0 to 4.0 ppm/° C. between −50° C. and 150° C. A thermal expansion coefficient, α1, of the multi-layer ceramic substrate as determined at a particular temperature and a thermal expansion coefficient, α2, of a to-be-tested silicon wafer as determined at the same temperature silicon satisfy a relation: 0 ppm/° C.