Traditional Japanese construction worker clothes are called tobi shouzoku. The men and women who put up Japan’s buildings have been getting a lot more fashionable recently if these catalogs from a clothing company in Okayama Prefecture called Kaseyama (site is Japanese only) are any indication.
The baggy pants are called nikka-pokka-. The reason that construction workers wear them is that the loose fit allows them to stretch their legs out comfortably when climbing around on buildings. Some construction workers also consider them to make their jobs safer. The baggy pants tend to get caught on things, making them more careful when they are working in high places.
All these clothes are being modeled by real construction workers, and I read that you can see these clothes on construction sites, but it’s kind of hard to imagine. If anyone out there has a picture of someone wearing this kind of stuff on the job, I’d love to see it.
Regular tobu shouzoku run around 6,000-8,000 Yen for the pants and 4,000 to 7,000 yen for the tops, but these high fashion ones seem to be around 30,000 or more.