The bomb, hidden in a bicycle parked on Qambrani Road in the suburbs of Quetta, was triggered by a remote control when the police van was passing by.
A police inspector was killed instantly, officials said.
Policemen and passers-by were among the five injured.
No group claimed responsibility for the bombing though such attacks are usually blamed on Baloch nationalist groups.
Initial reports on TV news channels had said that two policemen were killed.