A micromechanical sensor for measuring millimetric wave or microwave power, which sensor comprises a wave line for conducting the millimetric or microwave power and a part arranged to move and a fixed electrode, in such a way that the capacitance (C) between the part that is arranged to move and the fixed electrode couples to the wave power advancing in the wave line. According to the invention, the capacitance (C) between the part that is arranged to move and the fixed electrode is divided into at least two portions (C/n), which are located at a distance from each other, in such a way that the wave power advancing in the wave line couples to the portions (C/n) of the capacitance (C) consecutively and experiences the inductive load between the portions (C/n) of the capacitance (C). Thus the frequency band of the sensor can be substantially broadened and the reflective coefficient can be kept reasonably small.