Purpose: To report a particular circumstance that led to the abnormal complication of choroidal detachment (CD)-induced secondary angle-closure after trabeculectomy with mitomycin C (MMC). Observations: An 82-year-old Japanese male patient with underlying chronic obstructive pulmonary disease and suspicion of ocular venous congestion in both eyes whom diagnosed as primary open-angle glaucoma with uncontrolled intraocular pressure (IOP) of his left eye then underwent an uneventful trabeculectomy with MMC. After the surgery, his left eye had high IOP with a shallow anterior chamber (A/C) but the bleb was hyperfiltration. The high CD was found by B-scan ultrasonography behind the iris and after conservative treatment, the CD was improved, A/C was deepened, and IOP was lower to 16 mmHg. Conclusion and Importance: CD-induced secondary angle-closure after trabeculectomy with MMC is a complication to be considered in patients with shallow A/C and high IOP. Fundus examination should be done to rule out this condition before any aggressive treatment as CD can resolve spontaneously with time. Clinicians should be aware of this condition especially in patients with any signs of ocular venous congestion as there have been few reports mentioned about the complication in the patients.