Delayed cervical lymph node metastasis (CLNM) is the most negative prognostic factor of tongue squamous cell carcinoma (SCC). This study analyzed the relationship between clinicopathologic factors, including anatomic invasive depth (AID), and CLNM. A total of 212 patients with clinically node-negative (cN0) tongue SCC who had undergone partial glossectomy through the mouth were eligible for this retrospective study. The deepest portions where tongue SCC cells invaded as determined by microscopic analyses were classified into 5 categories, including epithelial and submucosal tissue, lateral extrinsic tongue muscle (ETM), intrinsic tongue muscles (ITM), paralingual and sublingual spaces, and medial ETM according to AID. We examined the relationship between clinicopathologic factors including AID and delayed CLNM. Multivariate analysis demonstrated that AID was an independent predictive factor for delayed CLNM (P=0.0022; odds ratio=7.1). Deeper invasion than ITM, including ITM, paralingual and sublingual spaces, and medial ETM, had high sensitivity and negative predictive value for delayed CLNM (94.4% and 95.7%, respectively). Precise elucidation of AID may be useful for the preoperative decision for performing elective neck dissection. None of 11 patients in whom the deepest portion where tumor invaded to lateral ETM (according to AID) showed delayed CLNM, although tongue SCC T4a tumor is defined by the presence of invasion of cancer cells to ETM. Tumors with invasion to lateral ETM might have to be excluded from the pathologic T4a category.