The police found 27-year-old Tomohiro Iwazaki near the Tokyo concert venue with a bloody, 7.5cm-long folding knife close by.
Iwazaki admitted he had lost his temper. He'd sent a gift to pop star Mayu Tomita, 20, which she refused.
So, Iwazaki said to the officials, he attacked her.
"I sent a gift (to Tomita) but it was returned," Iwazaki told the police, according to the South China Morning Post. "I asked her why but she gave an evasive answer, so I became mad and stabbed her numerous times."
The pop star had two dozen cuts and stab wounds to her neck, back, arms and chest, reports the Japan Times. Tomita, who had previously alerted authorities that Iwazaki was stalking her, is unconscious in a coma and remains in critical condition.
The owner of a nearby restaurant said "a trail of blood and a bloodstained mask were found on stairs leading to the room where the concert was to take place," according to the Times.
Tomita is one of Japan's so-called idols, young pop stars who can skyrocket to fame, buoyed by throngs of adoring fans.
Perhaps the most famous of Japan's idol supergroups is AKB48, which now boasts more than a hundred women, mostly in their teens to early 20s. Watching an AKB48 performance - the combination of youth and fantasy bordering on lasciviousness - has been compared to viewing a Balthus painting.
The price of an idol's fame, however, often comes with hyper-restrictive contracts. Typical clauses can bar an idol from marrying or dating, the BBC reported in January. One industry executive told Japan Today that idols' schedules, moreover, are kept so tightly managed finding time for a romantic partner is prohibitive.
The goal is a curated sense of kawaii, of cuteness that is not simply cute. Kawaii is "not just something you love," University of Tokyo's Roland Kelts said in 2008. "It's something you want to protect."
When 20-year-old Minami Minegishi, an idol in AKB48, admitted to spending a night with her boyfriend in 2013, she shaved her head and offered a public apology for being "thoughtless and immature," according to the BBC.
Celebrity stalking is by no means isolated to Japan; as a recent example, sportscaster Erin Andrews was awarded US$55 million in a widely-publicised stalking case. But after Minegishi's apology, the Japan Times criticised idol fan culture as "institutionally incapable of dealing with independence in young women. It seeks out and fetishises weaknesses and vulnerabilities" as part of a manufactured narrative.
The attack on Tomita mirrors the assault on two members of AKB48, who were struck at a fan "handshake" event in May 2014. A 24-year-old man wielding a handsaw broke the right hands of Anna Iriyama, 18, and Rina Kawae, 19, before security could restrain him.
The Japan Times reports that someone thought to be Iwazaki had sent threatening Twitter messages to Tomita. "I will never forget that I was looked down upon by you," one read.
Police plan to charge Iwazaki, according to the Times, with attempted murder.