We examined the effectiveness of trabeculectomy in decreasing the slope of mean deviation (MD) in Japanese patients with progressive normal-tension glaucoma (NTG) at low intraocular pressure (IOP) levels. The charts of patients who had undergone initial trabeculectomy with adjunctive mitomycin C for progressive NTG with medically controlled IOP < 15 mmHg in 2010-2013 were retrospectively reviewed. Seventeen eyes of 13 NTG patients who had undergone at least 5 times of visual field (VF) examinations in both of preoperatively and postoperatively with postoperative follow-up of ≥ 2 years were enrolled. Preoperative and postoperative MD slopes were compared to evaluate the effectiveness of trabeculectomy in slowing progression of VF. Mean IOP (8.1 ± 2.9 mmHg) and number of IOP-lowering medications (0.8 ± 1.5) were significantly lower postoperatively than preoperatively (13.9 ± 0.9 mmHg; P < 0.001 and 3.0± 0.4; P < 0.0001). In total, 91.7% of eyes with single-digit IOP postoperatively showed improvement in MD slope, whereas only 20.0% of eyes with IOP ≥ 10 mmHg postoperatively showed the improvement. Three eyes (17.6%) showed a decrease in visual acuity (VA) of ≥ 0.1 unit; this group had a lower mean postoperative IOP (6.0 ± 1.0 vs. 8.6 ± 3.0 mmHg; P = 0.1717) and a higher mean IOP reduction rate (56.2 vs. 38.5%; P = 0.8296) than eyes with a VA decrease of < 0.1 unit or no change. Thus, in this analysis of Japanese NTG patients with medically controlled IOP < 15 mmHg, achieving an IOP < 10 mmHg with trabeculectomy was beneficial for reducing the VF progression rate in progressive NTG at low IOP levels. However, an IOP < 7 mmHg by surgery would be required careful attention to VA decline.