Tritium (T) retentions in tile gaps (side surfaces) of the first wall of JT-60U were measured by a tritium imaging plate technique (TIPT). For all first wall tiles measured here, the T retention decreased from the front (entrance) to the bottom of the side surfaces showing superposing two exponential decays, which were already observed in the divertor region. Heavier erosion on the plasma-facing surface resulted in higher T retention in the front-side surfaces in the vicinity of the plasma-facing surface. In addition, wider gap width also resulted in higher T retention in the bottom side surfaces. Using the TIPT results, overall T retention in the side surfaces of the whole first wall was estimated to be [approximately]6 × 1017 T atoms, which was only one-tenth of total T retention in the plasma-facing surface of the first wall in JT-60U.