This paper reports the strength of a pseudorandom number generator MUGI, which was published as a stream cipher by Hitachi, Ltd. in 2001, against linear cryptanalysis. MUGI is one of the recommended ciphers of CRYPTREC, which is a project for the e-Government in Japan. It has two internal states called state and buffer, which are updated by a linear function λ and a non-linear function ρ. The non-linear function ρ and the linear function λ have already been analyzed, independently. In this paper, whole MUGI is analyzed by truncated linear cryptanalysis. The analysis of λ function is based on the state variables method. The result is combined to the result of the analysis of ρ function to make a trellis diagram. Viterbi search is conducted on the diagram to find the best possible linear path under 64-bit truncated linear cryptanalysis. As the result, the upper bound of the maximum linear characteristic probability is estimated as less than 2-138. Therefore, MUGI is secure against linear cryptanalysis.