Reduced time evolution of a two-level system is studied by means of an exactly solvable dephasing model. It is assumed that the two-level system is influenced by a disturbed Ohmic reservoir which is out of thermal equilibrium. The disturbance is caused by an interaction between a thermal reservoir and an ancillary two-level system before a system of interest interacts with the thermal reservoir. It is shown that the reduced time evolution of the two-level system can be non-Markovian even when it is Markovian in the absence of the disturbance. This means that the disturbance in the thermal reservoir can induce the non-Markovian time evolution of the two-level system. Dependence of the reduced time evolution on system-reservoir coupling strength, Ohmicity parameter, and reservoir temperature is examined in terms of coherence measure and trace distance.