A geometric mass concerning supersingular abelian varieties with real multiplications is formulated and related to an arithmetic mass. We determine the exact geometric mass formula for superspecial abelian varieties of Hilbert-Blumenthal type. As an application, we compute the number of the irreducible components of the supersingular locus of some Hilbert-Blumenthal varieties in terms of special values of the zeta function.2000 Mathematics subject classification: primary 11F41; secondary: 11E41.