This paper studies the linear cryptanalysis of the MBAL (MultiBlock ALgorithm) Cipher. MBAL includes SXAL8 Cipher and 3-round Fm function. Moreover, the size of the input data is variable. In this paper, we propose the strategy of making linear approximations for MBAL which are independent of SXAL8 and the size of the input data. We succeed in making an approximation with bias 2–5.49 from the first to second (or second to third) rounds, where the bias is defined as |Prob – 1/2|. Next, we show how to reduce the number of “effective key bits” from 96 to 80 in the first (or n-th) round. Finally, we succeed in making an approximation for MBAL with bias 2–16.57, which gives 1 bit of information on the key. © 1999 Scripta Technica, Electron Comm Jpn Pt 3, 82(10): 1–8, 1999