  We study the origin of gamma rays from the supernova remnant (SNR) RX
J1713.7-3946. Using an analytical model, we calculate the distribution of
cosmic rays (CRs) around the SNRs. Motivated by the results of previous
studies, we assume that the SNR is interacting with two-phase interstellar
medium (ISM), where dense clumps are surrounded by tenuous interclump medium.
We also assume that only higher-energy protons (~> TeV) can penetrate the dense
clumps. We find that pi^0-decay gamma rays produced by protons reproduce the
observed gamma-ray spectrum peaked at ~TeV. On the other hand, it has recently
been indicated that the observed ISM column density (N_p), the X-ray surface
brightness (I_X), and the gamma-ray surface brightness (I_g) at grid points
across the SNR form a plane in the three-dimensional (3D) space of (N_p, I_X,
I_g). We find that the planar configuration is naturally reproduced if the ISM
or the CR electron-to-proton ratio is not spherically uniform. We show that the
shift of the observed data in the 3D space could be used to identify which of
the quantities, the ISM density, the CR electron-to-proton ratio, or the
magnetic field, varies in the azimuthal direction of the SNR.
