Along some subduction plate boundaries, slow deformation is observable as seismically detected deep low-frequency tremor and geodetically detected slow slip events. These phenomena are considered as different manifestations of slow earthquakes characterized by fairly constant seismic moment rate. This paper presents a simple model of slow earthquakes that can explain wide variety of observed features including the steady moment rate and scaled energy, characteristics of tremor signals both in time and frequency domains, and the migration of the source location. In this model, slow earthquakes are represented as shear slip on circular faults whose radius is a random variable that is governed by a Langevin equation and three parameters, a diffusion coefficient, a damping coefficient, and a slip rate coefficient. This model expands on a previous scaling law for the slow earthquakes by providing a specific image of kinematics. Allowing for spatial variations of the parameters could potentially explain differences in behavior of slow slip events worldwide.