Fires broke out following the incident in the northern city of Sapporo, Kyodo news agency said, and images from the scene showed large flames and plumes of smoke rising in the night air.
A police official for the Hokkaido region told AFP “there are 42 injuries” but no one was killed.
They are still investigating the cause which “would take some time”.
National broadcaster NHK said the flames were extinguished several hours after the fire broke out, with police and fire department officers still at the site.
Earlier police warned of secondary explosions, according to Kyodo, which described buildings collapsing following the incident around 8.30 pm