A programmable memory device includes a plurality of one-time programmable (OTP) memory units, a search unit, a writing unit, and a reading unit. Each OTP memory unit is assigned an address. The search unit searches for the first writable OTP memory unit from the plurality of OTP memory units in a writing operation, or searches for the last programmed OTP memory unit from the plurality of OTP memory units in a reading operation. The writing unit writes data to be written and the bit length of the data to the first writable OTP memory unit. The reading unit sequentially reads data from the last programmed OTP memory unit.