  Comets are thought to have information about the formation process of our
solar system. Recently, detailed information about comet
67P/Churyumov-Gerasimenko has been found by a spacecraft mission Rosetta. It is
remarkable that its tensile strength was estimated. In this paper, we measure
and formulate the tensile strength of porous dust aggregates using numerical
simulations, motivated by porous dust aggregation model of planetesimal
formation. We perform three-dimensional numerical simulations using a monomer
interaction model with periodic boundary condition. We stretch out a dust
aggregate with a various initial volume filling factor between $10^{-2}$ and
0.5. We find that the tensile stress takes the maximum value at the time when
the volume filling factor decreases to about a half of the initial value. The
maximum stress is defined to be the tensile strength. We take an average of the
results with 10 different initial shapes to smooth out the effects of initial
shapes of aggregates. Finally, we numerically obtain the relation between the
tensile strength and the initial volume filling factor of dust aggregates. We
also use a simple semi-analytical model and successfully reproduce the
numerical results, which enables us to apply to a wide parameter range and
different materials. The obtained relation is consistent with previous
experiments and numerical simulations about silicate dust aggregates. We
estimate that the monomer radius of comet 67P has to be about 3.3--220
$\mathrm{\mu m}$ to reproduce its tensile strength using our model.
