  The folding of the triangular lattice embedded in two dimensions (discrete
planar folding) is investigated numerically. As the bending rigidity K varies,
the planar folding exhibits a series of crumpling transitions at K \approx -0.3
and K \approx 0.1. By means of the transfer-matrix method for the system sizes
L \le 14, we analyze the singularity of the transition at K \approx -0.3. As a
result, we estimate the transition point and the latent heat as K=-0.270(2) and
Q=0.043(10), respectively. This result suggests that the singularity belongs to
a weak-first-order transition.
