For the study of parity mixing in the compound states, a method to measure the longitudinal analyzing power A z of the isobaric analog resonance (IAR) as a function of energy in the proton elastic scattering is discussed. A thick target is used and energy loss of the protons in the target is utilized to measure the energy dependence. The energy resolution is presented and the sources of the systematic errors in A z are discussed together with the way of their elimination. The method has been shown to be quite efficient for deriving both the transverse and the longitudinal analyzing powers. The measurement of A z for the s-wave IAR at E cm =5.86 MeV in 90 Zr(p,p) and the analysis to derive PNC matrix elements are described.