To provide a building diagnostic device for easily acquiring earthquake-proof performance of a building including earthquake-resistance strength.

The building diagnostic device is combined with a shaker 20 that is disposed in a building to shake the building and a first acceleration detector 21 disposed in the building. The building diagnostic device changes a vibration frequency by the shaker and at the same time receives an acceleration detection signal obtained by the first acceleration detector during horizontal vibration, detects the oscillation frequency where the acceleration detection signal indicates a peak value as a dominant frequency f0 of the building, and includes an analyzer 15 for calculating earthquake-resistance strength TK of the building from a first predetermined formula TK=(2f0)2/g (g is an acceleration of 980 cm/sec2 of gravity) by the detected dominant frequency f0.