In general, substances expand by being heated and, in particular, it is known that organic resin materials have large linear thermal expansion coefficients. For example, in the case where a member made from an organic resin material is used in a device typified by a precision optical system or the like, a large change in size of the member due to a temperature change may cause deviation in the position of the optical system. In the case where a member used for the precision optical system is produced from only the organic resin material, it is desired that the linear thermal expansion coefficient thereof is 20×10−6/° C. or less.
In a method which is known as a method to solve this problem and which is disclosed in PTL 1, an organic-inorganic composite material is produced by adding an inorganic material, e.g., inorganic particles, in an organic resin material, so as to decrease the linear thermal expansion coefficient of the composite material. Concretely, it is disclosed that an inorganic filler, e.g., SiO2, Al2O3, or MgO, having a number average particle diameter of 1 nm or more and 100 nm or less is mixed into a thermosetting resin, e.g., an epoxy resin or a phenol resin. Furthermore, it is also disclosed that a resin composition having a linear thermal expansion coefficient of 20×10−6/° C. or less is thereby formed.