To obtain the subject stabilized enzyme for desulfurization etc. of gasoline, gas oil, etc. by immmobilizing an enzyme such as a peroxidase or a sulfinase in a structural unit having a size nearly coincident with that of the enzyme and structural stability.

This stabilized enzyme is composed of an immobilized enzyme for reforming a fuel and obtained by immobilizing at least one kind of enzyme selected from a peroxidase, a sulfinase, a monooxygenase, a dioxygenase and an isomerase in a structural unit having a size nearly coincident with that of the enzyme and structural stability and formed as pores capable of accommodating the enzyme in a mesoporous silica porous body. The immobilized enzyme is charged into various kinds of fuels such as gasoline or gas oil and capable of carrying out the fuel reforming such as desulfurization, an improvement in the octane number of the gasoline, an improvement in the cetane number of the gas oil, a reduction of the aromatic content, lowering of the boiling point or removal of impurities by enzymic actions thereof and stably maintaining the enzymic activity even in the fuels.