We propose to perform neutron powder diffraction experiments on the frustrated triangular lattice anitferromagnet AgFe1-xAlxO2 on WISH. Recently, we have studied magnetic orderings and dielectric properties in the undoped AgFeO2, which exhibits cycloid magnetic ordering concomitant with a ferroelectric polarization. [N. Terada et. al, PRL 109 097203 (2012)] The polarization can be understood by the extended inverse Dzyaloshinskii-Moriya mechanism. For further understanding of the theory, additional experimental facts to compare different magnetic orderings with ferroelectricity are necessary. The doped systems can be expected to show drastic change in magnetic ordering/symmetry due to disturbing delicate balance of strongly frustrated exchange interactions in AgFeO2. In this study, we would like to investigate how magnetic orderings are changed by nonmagnetic impurity doping.