The z-transform technique is used to investigate the model for
distribution of high-tax payers,
which is proposed by two of the authors (K. Y and S. M)
and others [12]--[14].
Our analysis shows an asymptotic power-law of this model with the
exponent $-5/2$ when a total ``mass'' has a certain critical value. Below
the critical value, the system exhibits an ordinary critical behavior,
and scaling relations hold.
Above the threshold, numerical simulations show that a power-law
distribution coexists with a huge ``monopolized'' member.
It is argued that these behaviors are observed universally
in conserved aggregation processes, by analizing an extended model.