We study the decretion disks formed around Be stars and characterized by the outflow of matter, unlike accretion disks. It is shown that, despite the opposite direction of the mass flow, the evolution of a large-scale magnetic field in such disks can be described within the framework of a large-scale dynamo approach. Field generation is considered a result of the combined action of the alpha effect and differential rotation. Peaceman - Rachford method was used to numerically solve the dynamo equation in a thin disk. Based on the observational data, a module simulating periodic emissions of substance and frozen-in magnetic fields was integrated into the computational code. The simulation is quite similar to that for accretion disks. Its results confirm the possibility of magnetic field generation in decretion disks.