Abstract Magnetoreflection spectra in the Faraday configuration have been investigated up to 40 T at 4.2K for a single crystal of (C 10H 21NH 3) 2PbI 4, which consists of two-dimensional networks of corner-sharing octahedra [PbI 6] 4-. The Zeeman shift is observed, and the effective g-factor is 1.42±0.04. The observed diamagnetic shift of the lowest exciton is (7 ± 4) x 10 −8eV/T 2, from which the two-dimensional Bohr radius is estimated to be 7 Å at most. It suggests that the exciton wave function extends only to the 4 nearest neighbor octahedra [PbI 6] 4- in the two-dimensional layer. By referring to the resonant Raman scattering spectra and reflection spectra of strained single crystals, a few narrow dips in high reflectivity region of the lowest exciton are ascribed to the vibronic excitons.