Actuators used in RF-MEMS tunable capacitors have an issue of creep-induced deformation. The creep is caused by a ductile-metal beam which is indispensable to attain the low loss. To avoid this issue, we previously reported an actuator structure that uses a brittle material, silicon nitride (SiN), at the stress-concentrated spring portions . The present paper aims to clarify a long-term creep immunity of the actuator. We first determined parameters of Norton's law by measurements and then carried out Finite Element Method (FEM) simulations. As a result, we found that the shift of the up-state capacitance is 2.2% after keeping the actuator in down-state position for 3 years at 85°C.