A method using BAS-MS type imaging plates (IPs) has been developed to estimate the values of ambient dose equivalent of scattered photon radiations in a diagnostic X-ray room where interventional cardiology procedures are frequently performed. An IP made of europium-doped BaFBr, a photostimulated luminescence material, is a highly sensitive two-dimensional radiation sensor. IP can be used as a passive dosemeter for a specific period, combined with a post-irradiation annealing procedure. The detector response to X-rays with effective energies between 30 and 120 keV were investigated via measurements with and without the use of metal filters (aluminum, copper, and cadmium) of varying thicknesses. The ambient dose equivalent can be evaluated by using a flat energy response to the dose equivalent obtained by taking the weighted sum of the sensitivity data measured under three different metal filters, without requiring knowledge of the effective energies of scattered photon radiations. The value of ambient dose equivalent at several measuring points in a diagnostic X-ray room was evaluated for periods of either 2 or 4 weeks using the flat ambient dose equivalent response and the annealing procedure. This method using IPs is practical for estimating values of ambient dose equivalent in a diagnostic X-ray room.