An apparatus for measuring radon and thoron using an ionization chamber is proposed. The apparatus includes: a pump for air inflow suctioning and sending external air to at least one channel; a first sensor module outputting an alpha particle detection signal of an electrical signal by detecting alpha (α) particles discharged from radon and thoron; an air inflow delay module delaying air for a predetermined delay time and then outputting the air; a second sensor module outputting an alpha particle detection signal of an electrical signal by detecting alpha (α) particles discharged from radon and thoron; and a control module discriminating normal or abnormal alpha particle detection signals, counting the normal alpha particle detection signals discriminated for a predetermined measurement time, and calculating radioactive ray concentration values on the basis of the counted number of times of the normal alpha particle detection signals.