An angular-multiplexing holographic memory system is one candidate for future optical data storage systems because of capability for abilities of “high-density recording” and “high-speed recording and reproduction”. We have developed a high-density recording method by reducing the hologram size in a disc to half by 2-level run-length-limited (RLL) modulation. In addition, to achieve further high-density recording, we introduce 4-level recording. However, 4-level RLL modulation requires high computational complexity in general. Therefore, we developed the dual 2-level RLL modulation method that can suppress increase in computational complexity and has good compatibility with the conventional holographic data storage system (HDSS). The high-density effect of the method was experimentally confirmed by the Fourier images and SNR of reproduced images. Consequently, the dual 2-level RLL modulation enables a doubling of the recording density, this gave an inspection for realizing a next-generation HDSS with the recording density of 4.8 Tbit/in.2.