We devised a simple method using a TaqMan fluorogenic probe for detection of a prevalent G6PT1 mutation W118R among Japanese patients with glycogen storage disease type Ib. The W118R mutation was detected in three of six newly diagnosed Japanese patients. The W118R-negative alleles were screened for causative mutations by sequencing analysis, revealing five novel mutations. The genetic tests using the simple TaqMan method coupled with sequencing analysis would facilitate the early diagnosis of this disorder.