The Ifo institute's closely watched business climate index eased slightly to 108.5 points this month from 108.6 points in April, the think tank said in a statement.
It was the first time since September that the index has fallen, but analysts had been expecting a slightly bigger drop.
Hence, Ifo president Hans-Werner Sinn insisted that the development was no cause for concern.
"Companies were once again more satisfied with their current business situation, but expressed slightly less optimism about the months ahead. The German economy remains on track," he said.
The sub-index measuring current business rose to 114.3 points, the highest level since June 2014, while the outlook sub-index slipped by 0.4 point to 103.0 points, the institute said.
Analysts generally viewed the data as positive.
"Germany has reached cruising altitude," said Berenberg Bank economist Christian Schulz.
"Overall, the Ifo index signals solid growth in the eurozone's largest economy, but also that after the rally in confidence indicators since the autumn 2014, the room for further improvement is now limited," he said.
That was backed up by a detailed breakdown of first-quarter gross domestic product (GDP) data released earlier by the federal statistics office Destatis.
According to the stats office, economic growth of 0.3 per cent in the first three months of this year was driven overwhelmingly by domestic demand, with foreign trade actually acting as a drag on growth.
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