  We show that the ability to probe primordial non-Gaussianity with cluster
counts is drastically improved by adding the excess variance of counts which
contains information on the clustering. The conflicting dependences of changing
the mass threshold and including primordial non-Gaussianity on the mass
function and biasing indicate that the self-calibrated cluster counts well
break the degeneracy between primordial non-Gaussianity and the observable-mass
relation. Based on the Fisher matrix analysis, we show that the count variance
improves constraints on f_NL by more than an order of magnitude. It exhibits
little degeneracy with dark energy equation of state. We forecast that upcoming
Hyper Suprime-cam cluster surveys and Dark Energy Survey will constrain
primordial non-Gaussianity at the level \sigma(f_NL) \sim 8, which is
competitive with forecasted constraints from next-generation cosmic microwave
background experiments.
