To provide a porous titania fine particles/clay composite material having high photocatalyst activity, high durability in an acidic solution and a special specific surface area measured by a BET method (using Brunauer- Emmett-Teller's equation) in a nitrogen gas adsorption method and a method for increasing crystallinity of the titania particles in the composite material by specifying an average pore diameter measured.

This method highly crystallizes amorphous or low crystalline titania fine particles in a titania fine particles/clay composite material to an anatase type by a hydrothermal treatment and provides a porous crystalline titania fine particles/clay composite material having ≥100m2/g specific surface area measured by a BET method. Used raw material clay is a mixture of at least one kind of montmorillonite as a swelling clay, smectite such as hectorite, vermiculite, synthetic mica and a fluorine-substituted analogous material, etc. A titania sol is reacted with clay to compound titania with the raw material clay. An average pore diameter as a measure of an interlaminar gap in the composite material does not exceed 15nm even after performing hydrothermal treatment at 250°C for several days.