At around 3:45 a.m., an unknown person drove the truck, loaded with gravel, backwards into the entrance of the office, located in the Hitotsuya area.
Police arriving at the scene found the truck abandoned with its bed tipped up and the gravel in a large mound in the street out front.
The impact into the structure heavily damaged the front door and a brick wall, police said.
The office, located in a residential area about 500 meters from Rokucho Station on the Tsukuba Express, belongs to a second-tier gang of the Yamaguchi-gumi that is based in Shizuoka Prefecture.
Police suspect that the incident is related to a dispute between the gang and the Matsuba-kai, another criminal syndicate.
Police are searching for the perpetrator on suspicion of causing