Spin-1 Bose gas with anti-ferromagnetic interactions can form a Bose condensate of singlet pairs (singlet molecules) at low concentrations of atoms. We show that despite the effective attractive inter-atomic interactions in spin-singlet channel, the molecular condensate can be stabilized against further collapse due to the repulsive interactions in the spin-2 channel, and determine the region of stability of the singlet condensate. Our results provide an example of stabilization of the Bose-condensed phase by competing attractive and repulsive interactions.