The attack on Saturday - the third on real estate firms in the past week - did not cause any casualties.
The interior ministry of the Basque regional government pinned the blame on armed Basque separatist group ETA (Euzkadi Ta Askatasuna), adding that the explosive device had contained around 1kg of explosive.
As well as being afflicted by ETA violence, the northern Basque region also suffers occasional bouts of street violence by disaffected youths.
A government statement urged ETA, which has killed more than 800 people in a four-decade independence campaign, to abandon violence and "leave in peace" the Basque people.
In recent weeks, French and Spanish police have arrested a string of suspected ETA members and last weekend French police said they had uncovered the biggest-ever haul of weapons belonging to the group in south-west France.