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To estimate subcriticality of a system, a new method, Indirect Bias Estimation Method, is proposed. Instead of comparing a calculated keff with a "measured" one which cannot be measured directly, a bias in calculated keff is evaluated by direct comparison between measured and calculated quantities which are measureable in subcritical experiments. With these errors of measurable quantities, accuracy of a subcriticality calculation is indirectly derived. Three estimation methods based on the source multiplication method, the pulse neutron method, and the exponential experiment method are presented. For three experimental methods, neutron count rate distribution, prompt neutron decay constant, and spatial decay constant are compared between calculation and measurement, respectively, deriving biases in calculated keff's. With biases obtained by this procedures, namely Indirect Bias Estimation Method, subcriticality of a system can be estimated more precisely than by "measured" keff's.