Organic magnet [Ph(NH3)](18-crown-6)[Ni(dmit)2] is a candidate for an S 1/2 two leg spin ladder system [S. Nishihara, T. Akutagawa, T. Hasegawa, and T. Nakamura, Chem. Commun. (Cambridge) 2002, 408.]. In order to investigate the non magnetic impurity effect for this compound, we performed X-band ESR measurement of [Ph(NH3)](18-crown-6)[Ni(dmit) 2]1-x[Au(dmit)2]x. Non magnetic impurity concentration were estimated to be x 0.029. ESR measurements have been performed from T 4.5 K to 273 K using the Bruker ESR spectrometer. At T 4.5 K, the fine structure ESR signals were observed. These signals suggest the existence of ferromagnetically coupled S 1/2 dimers in the system.