Purpose To investigate how the central visual field would be changed after fornix-based trabeculectomy with mitomycin C in Japanese normal-tension glaucoma (NTG) patients monitored for more than 10 years including before surgery. Methods This is a retrospective cohort study. We identified twenty-eight eyes of 28 NTG cases regularly monitored for more than 5 years and examined static visual field (VF) tests for more than five times before and after fornix-based trabeculectomy (including combined surgery). Based on preoperative data for 6.3 years, we evaluated postoperative changes for 6.0 years in 10–2 VF and 30–2 VF. Results Six patients were male and 22 females, the mean age was 57.9 years, and the mean deviation was − 13.7 decibels. After surgery, mean IOP decreased from 13.9 to 9.0 mmHg ( P  ≤ 0.01), and medication score also did. The rate of 10–2 VF deterioration was significantly suppressed from − 1.0 dB/year preoperatively to − 0.4 dB/year postoperatively ( P  ≤ 0.01). And when 30–2 VF was divided into six relevant sectors, the rate of deterioration at cecocentral and arcuate areas of the superior hemifield was suppressed postoperatively ( P  ≤ 0.01 and P  = 0.042, respectively). Conclusion Based on long-term preoperative data, a significant positive change in 10–2 VF was observed after fornix-based trabeculectomy with mitomycin C in Japanese NTG patients with a mean IOP of 13.9 mmHg. Central vision could be expected to be maintained for NTG patients even if its preoperative IOP is low after successful trabeculectomy.