  The B-mode polarization of the cosmic microwave background on large scales
has been considered as a probe of gravitational waves from the cosmic
inflation. Ongoing and future experiments will, however, suffer from
contamination due to the B-modes of non-primordial origins, one of which is the
lensing induced B-mode polarization. Subtraction of the lensing B-modes,
usually referred to as delensing, will be required for further improvement of
detection sensitivity of the gravitational waves. In such experiments,
knowledge of statistical properties of the B-modes after delensing is
indispensable to likelihood analysis particularly because the lensing B-modes
are known to be non-Gaussian. In this paper, we study non-Gaussian structure of
the delensed B-modes on large scales, comparing them with those of the lensing
B-modes. In particular, we investigate the power spectrum correlation matrix
and the probability distribution function (PDF) of the power spectrum
amplitude. Assuming an experiment in which the quadratic delensing is an almost
optimal method, we find that delensing reduces correlations of the lensing
B-mode power spectra between different multipoles, and that the PDF of the
power spectrum amplitude is well described as a normal distribution function
with a variance larger than that in the case of a Gaussian field. These
features are well captured by an analytic model based on the 4th order
Edgeworth expansion. As a consequence of the non-Gaussianity, the constraint on
the tensor-to-scalar ratio after delensing is degraded within approximately a
few percent, which depends on the multipole range included in the analysis.
