Metabolic cost during walking is positively associated with exercise intensity. As a walking assistive device, one of the major goals should be the maximization of wearers' metabolic benefits. Toward this goal, this paper experimentally evaluates the influence of maximum assistive force (MAF) of an authors' soft robotic suit, which has been developed to assist hip flexion for energy-efficient walking of elderly persons in daily activities, on metabolic cost reduction. Experiment results show that, for a 79-years-old healthy male subject, the metabolic cost reduction rate of the soft robotic suit was not linearly correlated with the value of MAF in the condition of the robotic suit worn and powered on (PON) compared with that of worn but powered off (POFF). Instead, it is interestingly observed that the metabolic cost was significantly reduced by an average of -12 % in the PON condition with MAF that the subject felt most comfortable with (24.5 N), while the reduction rates were -0.8 % and -3.1 % in the PON condition with MAFs that the subject felt weak (15.7 N) and strong (29.4 N), respectively, showing no significant differences between the two conditions. The findings of this study are beneficial for the future development of soft robotic suits that assist hip flexion of elderly persons.