An extremely low-background beta-gamma coincidence spectrometer has been constructed with heavy mass and anticoincidence shieldings. The main detector consists of a large well-type NaI(Tl) scintillator and two semi-cylindrical GM counters fitted into the well. The geometry of the NaI(Tl) scintillator is 6.5″ ∅ × 4.0″ (well; 3.5″ ∅ × 2.5″). The crystal has a 5 mm thick quartz window and is mounted in a 0.8 mm thick vacuum-melted copper. The crystal is matched to a quartz made photomultiplier EMI 9530 QR (photocathode 5″ in diameter). The main counter system is surrounded with a large scale kerosine scintillation counter, which acts as a guard counter. The detector system is set inside a 25 cm thick iron room, which was lined with 0.2 cm thick OFHC and 0.5 cm thick lucite plates. The iron bricks and the skeleton were made before 1945. The whole assembly was built in an underground cell heavily shielded against major cosmic ray component with a rock cover of at least 100 mwe. The background counting rate was 0.050 cpm at the 511 keV and the counting efficiency was 11.9% in beta-gamma mode for the annihilation peak.