We propose invariant key generation method using iris image. In conventional iris recognition system, iris features are not consistent because most feature extraction techniques are sensitive to the variations of captured iris image data. In this paper, we introduce the invariant binary feature which is defined as iris key. Regardless of iris image variations, we can extract the invariant iris key in our experiments. Our method can be applied to various fields especially for key protection and key management application using biometric data, which include e-commerce, smart card, and logging into personal computer or secured web site. To generate the iris key, we propose the reference pattern which is designed as lattice structured image to represent a bit pattern as individual's identity. The reference pattern and iris image are linked into filter. In the filter, iris texture is reflected according to magnitude of iris power spectrum in frequency domain. There is a trade-off relation-ship between with-in class tolerance and between-class discrimination, so that we can control these factors which are included in the filter. In experimental results, we were able to tune the system to a False Rejection Ratio(FRR) level of 1.0%.