Thermal conductivity λ of gases was measured by a steady state method using a microbridge-type MEMS sensor fabricated on a Si substrate. The sensor consisted of a hot wire with two adjacent thermocouples (TCs) on the surface of a SiO2 microbridge. Due to the small heat capacity of the microbridge, the temperature after heating increased much faster to a saturated vale ΔT than the onset time of natural convection, enabling the ΔT to be used for steady-state analysis. In moving gases, the difference of the ΔT between the upstream and the downstream TCs was proportionally increased with the flow velocity. In static gases, this difference intrinsically became zero, and the heat flow QP from the hot wire to base SiO2/Si pad areas was calculated using the input heating power Q to yield a constant ΔT for a set of gases with known λ. Once QP was obtained, the λ of unknown gases was found to be easily estimated with errors of less than 1% by measuring the Q to yield the same ΔT.