The bomb went off near a bus station and civil hospital in the town of Rakhni, about 300 km north of Quetta, the provincial capital, reports said.
Five people were killed and 21 others wounded, some of them seriously, they said.
No one has claimed responsibility for the attack. But authorities blame such attacks on tribesmen who oppose the military's campaign to build new garrisons, which the government says are needed to bolster security.
Balochistan has been rocked by shootings, small-scale bombings and rocket attacks in recent years. Most have been blamed on tribesmen demanding increased royalties for gas extracted from the province.
