Gunma Insect World Insect Observation Hall (県立ぐんま昆虫の森 昆虫観察館)

Gunma Insect World (県立ぐんま昆虫の森), a prefectural park, covers 45 hectares in Kiryū (桐生), where the woods, fields, rice paddies and streams of a traditional countryside have been recreated. The basic development plan was published in April 1997; in July 1997 Gunma Prefecture commissioned Tadao Ando's office for the park's basic design and the detailed design of its outdoor grounds.

The insect observation hall, the centrepiece of the park, is by the same office. Work on its main building began in June 2002 and finished in September 2004; the park opened fully on 1 August 2005.

The building is set on a slope. The exhibition rooms are dug into the ground; above them is thrown a glass roof in the form of a portion of a sphere, part of which, separated from the rest by a glass wall, serves as the greenhouse. It covers 1,100 square metres and recreates the environment of Iriomote Island (西表島). The building has one basement and three storeys above ground, with 5,084 square metres of floor area; its structure combines steel and reinforced concrete, with a few steel-reinforced-concrete parts.

The frame of the glass roof is a single-layer steel lattice shell, a network of triangles 4.5 to 5 m on a side laid out on a sphere of 57.241 m radius and cut out by three great circles; the three corners of the shell are about 70 m apart in a straight line. It rests on those three corners, on box-section columns about 4 m apart that also act as mullions around the greenhouse, and on V-shaped steel columns around the perimeter. The inner beams are kept as slender as possible.

The glass is held by fittings that project from the steel members, using the DPG method, with as little material as possible.

The hall shows live animals of the traditional countryside and insects from around the world all year. In its insect-contact greenhouse, a waterfall runs among subtropical plants, and Idea leuconoe, one of Japan's largest butterflies, flies there with other species.

The building also has an insect-themed craft corner, a video hall, and a room for seasonal and special exhibitions. In the annex are a library of more than 10,000 books and a café-shop with about 40 seats.

The park is open from 9:30 to 16:30 from April to October and from 9:30 to 16:00 from November to March; the last entry is half an hour before closing and no reservation is needed. It is closed on Mondays (the next day when Monday is a public holiday), from 28 December to 8 January and while exhibitions are changed; it sometimes opens on Mondays during the spring holidays, Golden Week and the summer.

You enter the building at its highest level, where the reception desk is. There is a free car park with 300 spaces; the two nearest stations are 3 km away, about 10 minutes by taxi (on-demand taxi from one of them).