We constructed a new forward calorimeter which covers a small polar angular region in the TOPAZ detector at the TRISTAN e +e - storage ring of KEK. It comprises large BGO crystals with a silicon photodiode readout system for energy measurements, and silicon strip detectors for position measurements. It has been operating since November 1989. We present detailed design considerations and the performance from test beam measurements and e +e - colliding beam experiments. We also show the results of luminosity measurements and discuss the systematic errors.