The bomb went off at 7.25am near a bridge in Ban Sawor Hilae (Moo 6) in Tambon Sawor, as the last ranger passed within 10 metres of the explosion, which was focused in another direction.
Police officers inspecting the scene found a 30cm-deep, 1.2-metre-wide crater with bomb fragments.
They said they suspected the explosives were placed in a 10-kilogram metal box and detonated by radio.
Police believe the explosion was part of disturbances ahead of the New Year festival.