"The fact is that Japan detected the submarine's movement from the moment it departed from Qingdao. Tokyo kept a close eye on the submarine for 53 hours, knowing that it passed the island of Okinotorishima, reached the east side of Guam, and went through an ocean trench between Ishigaki and Miyako on its way back," Lo said.
Based on the movement of the submarine and the route it took, Japan believed the submarine must have taken several trips in its waters before.
"Otherwise it could not have possibly known the existence of the ocean trench between Ishigaki and Miyako," he added.
As the submarine travelled through the ocean trench, Japan demanded that it surface. But it would not heed the request.
"At that point, Tokyo decided to let its public know about the intrusion of the Chinese submarine," Lo said. "It wanted the public to know that China had gone too far."
Taipei did alert Tokyo of the presence of the submarine, Lo said, but later realized that Tokyo had known about the vessel's trip.
The submarine incident was just one event that has led to worsening Japan-China ties. In China, anti-Japan sentiment was fueled by former president Lee Teng-hui's (李登輝) trip to Japan last year and Koizumi's visits to the Yasukuni Shrine, a site that Japan's neighbors strongly associate with its militaristic past.
Lo noted that Taiwanese officials' visits to Japan should be seen as a normal event rather than Taiwan's deliberate move to provoke China.
"China cares very much about such symbolic gestures ... Beijing was unhappy that Japan told it about the decision to issue Lee a visa the day before Lee got his visa," he added.
Taiwanese and Japanese lawmakers should strengthen mutual exchanges, Lo said. During his stint in Japan, more than 250 members of the Diet -- Japan's parliament -- visited Taiwan.
The former envoy did not find Koizumi's visits to the war shrine disturbing.
"Koizumi made his thoughts very clear before he visited the site," Lo said. "He stressed that Japan should never repeat its militaristic past but said that he, as the prime minister, should pay homage to those who sacrificed their lives for Japan."
Koizumi would not be moved by China's demand not to visit the shrine. "He makes his own decisions," Lo said.
Citing out that a recent poll showed that more than 58 percent of the Japanese people disliked China, Lo explained that by contrast, ties between Taiwan and Japan are growing warmer. He said one of the main reasons is that both are democratic countries.
That's not to say that Taiwan and Japan don't have their differences, though. Lo said that the two countries should work together to set a temporary boundary on their economic waters so as to reduce fishing disputes.
The boundary between the two countries' economic waters remains undefined even though both sides have held more than 20 meetings on the issue.
Earlier this month, hundreds of fishermen gathered in front of Japan's representative office in Taipei to protest being arrested by the Japanese coast guard for fishing in an economic zone claimed both by Taiwan and Japan.
"Japan has settled economic waters boundaries with North Korea and China. We should at least set a temporary boundary with Japan," he said.
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