The invention allows for testing by high velocity signals of high density LSIs prior to being packaged having an electrode spacing on the odder of 150 .mu.m, for example. Coaxial transmission lines 13 for termination formed through a terminal support 11 are arranged in a two-dimensional array. Connected to one ends of the transmission lines 13 are one ends of contact pins 18 such as conductive whiskers while the other ends of the transmission lines 13 are connected to a transmission line block 61 of a three-dimensionally upwardly gradually broadening configuration through a connection plate 72 similar in construction to the terminal support 11. The transmission line block 61 holds high frequency transmission lines 62 for relay connected at one ends to the other ends of the coaxial transmission lines 13 and having spacings between the adjacent lines broadened at the other upper ends. The widely spaced upper ends of the transmission lines 62 are connected to a performance board (not shown). The contact pins 18 are about 0.3 to 0.5 mm in length and the characteristics impedance is maintained uniform over all the transmission lines 62. The terminal support 11 is replaceable.