A Poisson process is one of the fundamental descriptions for relativistic particles: both fermions and bosons. A generalized linear photon wave equation in dispersive and homogeneous medium with dissipation is derived using the formulation of the Poisson process. This formulation provides a possible interpretation of the passage time of a photon moving in the medium, which never exceeds the speed of light in vacuum. The motion of relativistic particles is represented by the Poisson process. Kac proposed the continuous version of Poisson process in 1956 [1]. Gaveauet al derived the Dirac equation in 1984 using the formulation of Poisson process [2]. McKeon and Old constructed a modified version of the Poisson process in 1992 [3], where the relativistic particles can move both forward and backward in time. These Poisson processes were extended in 2001 [4] to more general frameworks of three-dimensional Dirac equation including an external field. Bialynicki-Birula invented a linear photon wave equation in 1994 [5] as a linearized version of the d'Alembert wave equation. Sipe verified in 1995 [6] that this wave equation identifies the ordinary canonical quantization.We treat the case that a time-dependent external potential V´tµ exists [4]. Let an absorption probability in a time interval δ be I´tµδ , where the path can be absorbed at most once in a time interval δ by the influence of the potential at time t. The conditional expectation is represented aswhere 1 I δ corresponds to a time shift generator in a time interval δ , and the onedimensional space-shift generator is L c∂ ∂ x. The one-dimensional space-shift generator has to be generalized to a three-dimensional generator with minimal coupling, L c σ σ ¡´∇ ∇ ie A´xµ µ in the case that vector field A exists, where σ σ gives the Pauli 413