The Onnagawa shale is Miocene, bio-siliceous and known as the main source rock in Japan. Frequent and intensive oil shows are observed through the Onnagawa formati on around the Ayukawa field. The thickness of this layer is about 1,600 ft around this field. The porosity is around 30%. The Permeability ranges from 0.01 mD to 1 mD. Onnagawa is considered to be similar to Monterey shale. Acidizing in Monterey shale has a long history of success. Though the production is generally improved by removing the drilling and completion damage, unexpected favorable responses from low permeability rock are also reported. These improvements are considered to result from dissolving the calcite and clay minerals in natural fractures and enhancing the wellbore connection into natural fracture network. The acid stimulation test was conducted to verify this acidizing effect in Onnagawa shale. The target well showed very low productivity. The perforation interval is 40 m (130 ft) long. Weakly fracture distribution is detected from wellbore images. No skin and low permeability on the order of 0.01 mD were estimated from the well test analysis. So the objective of this stimulation is not the removal of drilling and completion damage, but the improvement of natural fracture conductivity. The stimulation program was comprised of three stages that alternated 15% HCl preflush, 12% HCl + 3% HF main acid, NH4Cl overflush, and particulate diverting agent. The volumes of preflush and main acid were 80 gallon/foot each. The significant pressure drops indicating the injectivity improvement were observed during both preflush and main acid injection. The oil production dramatically increased from 1.5 kl/day (10 BOPD) to 50.0 kl/day (315 BOPD). The productivity index also increase from 0.3 kl/day/MPa to 45.0 kl/day/MPa. This test is the first successful stimulation of the tight oil formation in Japan.