The purpose of this paper is to study an analogue of
Euler's constant for the Selberg zeta functions of a compact
Riemann surface and the Dedekind zeta function of an algebraic
number field. Especially, we establish similar expressions
of such Euler's constants as
de la Vall\'ee-Poussin obtained in 1896 for the Riemann zeta
function. We also discuss, so to speak, higher Euler's constants
and establish certain formulas concerning the power sums of essential
zeroes of these zeta functions similar to Riemann's explicit formula.