  We propose a new method for finding galaxy protoclusters that is motivated by
structure formation theory and also directly applicable to observations. We
adopt the conventional definition that a protocluster is a galaxy group whose
virial mass $M_{\rm vir} < M_{\rm cl}$ at its epoch, where $M_{\rm
cl}=10^{14}\,M_{\odot}$, but would exceed that limit when it evolves to $z=0$.
We use the critical overdensity for complete collapse at $z = 0$ predicted by
the spherical top-hat collapse model to find the radius and total mass of the
regions that would collapse at $z=0$. If the mass of a region centered at a
massive galaxy exceeds $M_{\rm cl}$, the galaxy is at the center of a
protocluster. We define the outer boundary of protocluster as the zero-velocity
surface at the turnaround radius so that the member galaxies are those sharing
the same protocluster environment and showing some conformity in physical
properties. We use the cosmological hydrodynamical simulation Horizon Run 5
(HR5) to calibrate this prescription and demonstrate its performance. We find
that the protocluster identification method suggested in this study is quite
successful. Its application to the high-redshift HR5 galaxies shows a tight
correlation between the mass within the protocluster regions identified
according to the spherical collapse model and the final mass to be found within
the clusters at $z=0$, meaning that the regions can be regarded as the bona
fide protoclusters with high reliability. We also confirm that the
redshift-space distortion does not significantly affect the performance of the
protocluster identification scheme.
