Work started on the Welsh Assembly Government building in 2007
A new £20m assembly government base in Aberystwyth has been officially opened by First Minister Rhodri Morgan.
The office will house up to 535 staff, including workers from the rural affairs, heritage and economy and transport departments.
Around 120 staff are relocating from other areas of Wales, with some new posts being created.
The new offices form part of plans to spread civil servants' jobs more widely around Wales.
The offices will house 460 civil servants by the turn of the year but could reach the total of 535 by spring of next year.
The first of three regional offices opened in Merthyr Tydfil in 2006 under the scheme.
A base in north Wales at Llandudno Junction is expected to open in 2010.
Mr Morgan said: "The opening of this new office in Aberystwyth heralds the latest step in our commitment to bringing our services even closer to the people of Wales.
"We have established one important base already in the heart of the valleys in Merthyr Tydfil. This one in Aberystwyth is in the heart of rural mid Wales and Llandudno Junction on the central junction of north Wales will be third.
"We are sending a clear message that we wish to engage more proactively with people on a local level and ensure they are better informed about our work. It also provides people with a better opportunity to tell us their views."
The new building also has a public visitors area called Y Bont - The Bridge.
Mr Morgan said he hoped many people would take advantage of this opportunity.
"This is about much more than just an office space. Y Bont is an innovative public area, the first of its kind within a government office.
"I really hope that people will see it as a place to call in, talk to staff and give us their thoughts."
The office has been designed with the environment in mind with power and heat being generated using wind turbines, solar panels and a biomass heater, which is shared with neighbours Ceredigion council.
A number of staff have relocated from assembly government offices in Aberystwyth to the new building.
There will be a public open day at the building on Saturday between 1000 and 1600 BST.