Captain Diwakar Ram (11th minute), Mandeep Antil (18th and 24th minutes), Mohammad Amir Khan [ Images ] (52nd and 70th) and Victo Singh (58th and 62nd) scored for the Indians.
For Japan, Harayuki Fujiyoshi (41st minute) and Kenta Tanata (65th) gave respectability to the scoreline at the Seng Kang Sports Complex.
With this victory, India have three points and will play Belgium on Monday.
Belgium defeated England [ Images ] 2-0 in another Group G encounter earlier in the day at Johor Bahru, Malaysia.
India began proceedings at a slow pace but soon accelerated and caught the Japanese defence on the wrong foot.
Diwakar opened the scoring with a crisp flick past Japanese goalkeeper Katsumi Ichikawa from India's second penalty-corner in the 11th minute.
India got two more penalty-corners in quick succession and right winger Mandeep made no mistake in deflecting home both the chances through indirect shots.
The Indians could have increased their tally before the changeover but Ichikawa's goalkeeping came to Japan's rescue.
After the change of ends, Japan reduced the margin against the run of play, with Fujiyoshi dodging past a host of Indian defenders and an off-guard Indian goalkeeper Mrinal Choubey to score a brilliant field goal.
India, however, came back strongly and dictated terms thereafter with some deft moves between their forwards. Amir Khan and Victo tore apart the rival defence and scored two goals each.
Japan scored another consolation goal when Tanata pushed past substitute goalkeeper Harjit Singh in the 65th minute from a penalty stroke after Diwakar deliberately stick checked him in the Indian circle.