Study of pressure effect of organic weak-ferromagnet 1,3,5-triphenyt-6-oxoverdazyl (TOV) with the transition temperature T-c(P-0) = 5.0 K (P-0; ambient pressure) has been motivated by the experimental finding that slight dilution of TOV with the non-magnetic derivative changed the Weiss temperature (Theta) from negative to positive, i.e. from antiferromagnetic (AF) to ferromagnetic (F). We understood that above change of Theta would originate from the slight change of the crystal structure by an internal stress. Expecting the pressurization would bring about the same result as the dilution effect, we measured the ac magnetic susceptibility of pure TOV under the pressure up to P = 10.9 kbar, The pressure effect of pure TOV, however, revealed no transition from AF to F, but did the enhancement of T-c(P). The pressure dependence of T-c(P) is characterized with T-c(P) = T-c(P-0) (1 + 0.086 P), and T-c(P) exceeds 10.0 K at P= 10.9 kbar.