There is provided a method of sealing and molding an optical device with resin by employing a die including a top piece, a bottom piece, an intermediate piece, and a mold release film pinched between the bottom and intermediate pieces and thus tensioned as prescribed to cover the bottom piece's cavity, when the bottom piece is heated, and the mold release film expands and thus closely contacts the cavity's entire surface along the cavity's geometry so that the optical device can be sealed in transparent set resin shaped as desired.