Visitor figures to Keighley's Cliffe Castle museum have plummeted by almost 20,000 in the last year.
And membership of the Friends of Cliffe Castle, who raise funds for the museum, has also fallen.
Latest figures show that last year 76,000 people attended the museum compared with 93,000 in 1998.
Bradford Council's recreation boss, Councillor Barry Thorne (Lab, Keighley West) said he was disappointed by the fall.
"But this is not a one off - the Bronte Parsonage Museum at Haworth has seen a fall over recent years and so has East Riddlesden Hall. There has been a fall in visitors to places throughout the country," he said.
"We need to keep people's interest - unless that is maintained then they stop coming to museums.
"But we're on the verge of a new era at Cliffe Castle with the Working Landscape's project which will see exciting changes and improve visitor numbers."
John Bell, chairman of Friends of Cliffe Castle, said the fall in membership was only 12 but it had been steady at about 150 for several years.
Income had also fallen and he believed that was caused by fewer people visiting the museum.
"I think there have been fewer school visits this year, possibly as a consequence of the upheaval in the education system," he said.
"But I'm optimistic about the future. There are a lot of good things happening like the Working Landscapes project."
Geology curator Alison Armstrong said the museum would be offering visitors more hands-on enjoyment.
"There will be computers, video microscopes and much more material which can be handled by visitors," she added.
The museum is to spend £750,000 creating state-of-the-art exhibits using computers and videos. It is expected to push visitor figures over the 100,000 mark.
The project - mainly financed by the National Lottery and including contributions from Keighley's Single Regeneration Budget, the Friends of Cliffe Castle, and the independent charity the Wolfson Foundation - has gone out to tender and work on the first of two phases is expected to start in the summer.
It will involve upgrading the museum's existing classrooms and exhibits and will focus on explaining how man has had an effect on his environment over the ages.
Converted for the new archive on 30 June 2000. Some images and formatting may have been lost in the conversion.
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