  We consider the large-scale cosmic structure generation by an inflation based
on a pure f(R)-type gravity theory. Comparison with recent CMBR observations
gives the following results: (1) The near Zel'dovich spectral conditions
uniquely choose R^2-type gravity. (2) The R^2 gravity predicts specific nearly
scale-invariant Zel'dovich spectra for both the scalar- and tensor-type
perturbations. Thus, (3) the considered model survives current observational
data. The COBE-DMR quadrupole data (4) give constraints on the coupling
constant and the energy scale during inflation, and (5) require the
gravitational wave contribution to be suppressed. (6) Therefore, future
observations of (2) and (5) can provide strong tests of the inflation scenario
based on R^2 gravity. Parallel analyses made in the conformally transformed
Einstein frame give the observationally identical results.
