Open-ended coaxial probe is used to measure complex permittivities of materials in a wide bandwidth. Complex permittivity measured by a conventional coaxial probe suffers from severe fluctuation due to instrumental uncertainty when the difference between reflection coeffcients of reference and measuring materials is small. In this paper, open-ended coaxial probe with protruding inner conductor is suggested to increase the difference between reflection coefficients of reference and measuring materials. Its validity is assured by FDTD simulation and actual measurement.