In this paper, we attempted to find in the brain the factors of the human side, which is responsible for the market mechanism called the “invisible hand” by Adam Smith, by using fMRI experiments. We tried to identify the activated part of the brain that the allocator determines the amount of money to be paid to the responder in playing the ultimatum game. That part is the part with which human being considers the other party’s request and considers also its own interests in the process of trial and error consisting of market mechanism. This is because it is thought to be responsible for the function. It is a brain region called the left supramarginal gyrus, and its activation degree has a significant correlation with the sympathy scale of psychology. Accounting information derived from acquisition cost basis is fundamental information for trial and error done in the brain, that is, for market mechanism.