More than 40 people were injured in an explosion on Sunday night at a restaurant in northern Japan, police said. The explosion occurred in Sapporo, the capital city of Japan's northern main island of Hokkaido, and caused nearby apartment buildings and houses to shake. Police said 42 people were injured. Most of the injuries were not severe mildly, but one person was left in a serious condition. The cause of the explosion, which took place in a two-storey restaurant in Sapporo's Toyohira district, is under investigation, police said. However, a witness told public broadcaster NHK that he smelt gas after the sound of an explosion. NHK footage showed the restaurant in flames, with debris scattered on the ground. <span class="captionwrapper" style="display: block;"><a class="p-smartembed" data-attr-caption="Firefighters work on site where a large explosion occurred at a restaurant in Sapporo, Japan, 16 December 2018. EPA/JIJI PRESS " data-attr-has-caption="true" data-attr-q="0.3" data-attr-w="400" data-onecms-id="policy:1.803455" data-onecms-type="image" href="javascript:window.parent.actionEventData({$contentId:'policy:1.803455', $action:'view', $target:'work'})" polopoly:contentid="policy:1.803455">Firefighters work at the site of the damaged restaurant in Sapporo.</a> EPA</span> The footage later showed grey smoke billowing from the restaurant as dozens of firefighters poured water on to the building, which was charred and close to collapse. Windows on an apartment building next door were broken, and cars parked outside were partially covered with debris.