Three-dimensional photonic crystals with a diamond structure, which are composed of the TiO2-based ceramic particles dispersed in an epoxy lattice, were fabricated by stereolithography. The diamond structure showed a photonic band gap in the 14.3–17.0 GHz range along the Γ-K 〈110〉 direction, which is close to the band calculation using the plain wave expansion method. Two types of lattice defects—air cavity and dielectric cavity—were introduced into the diamond structure by removing a unit cell of diamond structure or inserting a block of the lattice medium into the air cavity. The transmission of millimeter waves affected by multiple reflections at the defects was measured in the photonic band gap. Resonant frequencies in the defects were calculated and compared with the measurement results.