The incident reportedly took place at around 3:15 p.m. Images and videos posted on social media show a fire on the second floor of the building and fragments of glass scattered across the road.
The Tokyo Metropolitan Police are investigating.
Bystanders describe scene
A man who posted a video of the fire on social media spoke to NHK.
"I suddenly heard an explosion so I looked on the road and there were things scattered everywhere," he said. "Smoke started gradually rising up after that."
A manager of a nearby Italian restaurant said he heard the sound of the explosion from inside.
"I was taking a break when I heard a sound and thought there might be an earthquake," he said. "Then I went outside and saw a fire on the second floor of the building."
The police say four people are being treated for minor injuries. Two are employees from the restaurant where the explosion is believed to have taken place, while the other two are passersby.
Explosion took place at bar
The explosion occurred in a bar on the second floor of the building. Those injured include a manager in his 50s and an employee in her 50s.
The manager told police the explosion happened when he lit a cigarette lighter in the smoking area of the bar. Police are investigating the cause.
The building is eight stories tall, with one basement floor, and is made of reinforced concrete. It is located about four minutes from Shimbashi Station.
The bar was reportedly preparing to open for the evening at the time of the incident. According to its website, it is open from 4 p.m. to 2 a.m. on weekdays.
The bar is frequented by people who enjoy horse riding and racing, with the website describing it as "a place that connects peoples, horses, and dreams."
Expert: Cause could be gas leak
Professor Sekizawa Ai of the Tokyo University of Science, an expert on fire prevention in buildings, says a gas leak could be the cause of the explosion.
"Judging from the situation at the scene, some kind of gas leaked into a room on the second floor and triggered an explosion," he said.
He added, "It seems there is no damage on the other floors and the explosion is not believed to have filled the building with gas."
But he warned that people should not approach the site until the cause has been confirmed.