1. An external condition control device based on measurement of brain functions to control an external device to match changes in blood amount at a number of sites on a brain of a subject acquired by near infrared spectroscopy by using a multi-channel fiber holder installed on outer side of a head of the subject the external condition control device comprising: a plurality of samplers receiving measurement data of the blood amount measured at the sites of the brain; a plurality of primary agents receiving information of the blood amount at the sites of the brain from the plurality of samplers, converting the information of the blood amount to primary output values; a plurality of secondary agents receiving the primary output values, converting the primary output values to secondary output values; a plurality of tertiary agents converting the primary output values and the secondary output values to tertiary output values by using learning parameters obtained from a synthesizer; the synthesizer integrating the secondary output values and the tertiary output values based on a weighted linear sum of the secondary output values and the tertiary output values and outputting a result of integration to control the external device; a medium storing weights used for the weighted linear sum at the synthesizer and statistical data of output value history of each of the plurality of primary, secondary, and tertiary agents; a measurement phase mode operation means for providing an introspective theme to be executed by the brain and measuring the blood amount by using a current setting of the fiber holder while the brain executes the introspective theme; and an evaluation means for comparing the statistical data of the output value history and an output value in the measurement phase mode for each of the plurality of primary, secondary, and tertiary agents, and calculating a reliability value for each of the plurality of primary, secondary, and tertiary agents in the current setting of the fiber holder, said evaluation means compares the output values of the measurement phase mode with the past statistic values for every agent of the plurality of primary, secondary, and tertiary agents, and calculates the reliability values for every agent of the plurality of primary, secondary, and tertiary agents, wherein the weights used for the weighted linear sum at the synthesizer are determined based on the reliability value for each of the plurality of primary, secondary, and tertiary agents in the current setting of the fiber holder.