Roller is submerged in water. Roller has mass m. Density of water is known and its value is ρ. When roller is in equilibrium position it is submerged in water on depth x. Two forces work on roller: gravity force and buoyancy force. We assume that there is no viscosity between roller surface and water. We also assume that there is no energy dissipation. In one moment roller was moved to bigger depth from its equilibrium position by distance Δx. After precipitation from equilibrium position roller executes harmonic vibrations in water. Subject of example is to find roller's harmonic vibrations period.