An adjustable probe device includes fixed and movable probes, at least one of which is a signal probe having a coaxial structure. The movable probe is linearly slidable with a ground unit thereof abutted against a ground unit of the fixed probe. Another adjustable probe device includes a first movable probe for being grounded, and fixed and second movable probes both having a coaxial structure. Any of the two movable probes is selectable to be a functioning probe in a way that the contact ends of the functioning and fixed probes are located on a same plane for contacting two pads of a DUT at the same time, and the functioning probe is linearly slidable with a ground unit thereof abutted against a ground unit of the fixed probe. As a result, the probe interval is adjustable, lowering the cost of the impedance testing apparatus for circuit boards.