  Hand crumpled paper balls involve intricate structure with a network of
creases and vertices, yet show simple scaling properties, which suggests
self-similarity of the structure. We investigate the internal structure of
crumpled papers by the micro computed tomography (micro-CT) without destroying
or unfolding them. From the reconstructed three dimensional data, we examine
several power laws for the crumpled square sheets of paper of the sizes
$L=50\sim 300$ mm, and obtain the mass fractal dimension $D_M = 2.7\pm 0.1$ by
the relation between the mass and the radius of gyration of the balls, and the
fractal dimension $2.5\lesssim d_f \lesssim 2.8$ for the internal structure of
each crumpled paper ball by the box counting method in the real space and the
structure factors in the Fourier space; The data for the paper sheets are
consistent with $D_M = d_f$, suggesting that the self-similarity in the
structure of each crumpled ball gives rise to the similarity among the balls
with different sizes. We also examine the cellophane sheets and the aluminium
foils of the size $L=200$ mm and obtain $2.6\lesssim d_f\lesssim 2.8$ for both
of them. The micro-CT also allows us to reconstruct 3-d structure of a line
drawn on the crumpled sheets of paper. The Hurst exponent for the root mean
square displacement along the line is estimated as $H\approx 0.9$ for the
length scale shorter than the scale of the radius of gyration, beyond which the
line structure becomes more random with $H\sim 0.5$.
