Studies of system identification (SI)-based structural health monitoring (SHM) are being actively conducted to ensure structural safety. Recently, many SI techniques have been developed using the output-only SI paradigm for estimating the modal parameters. The features of these output-only SI methods are obtained using frequency domain decomposition (FDD) and stochastic subspace identification (SSI), both of which involve the use of algorithms based on orthogonal decomposition, such as singular value decomposition (SVD). However, the SVD leads to a high level of computational complexity to estimate modal parameters. This paper proposes a technique to estimate the mode shape at lower computational cost. This technique shows pseudo-modal responses (PMR) through a bandpass filter and suggests a time-history modal assurance criterion (MAC). Finally, the mode shape is estimated from the PMR and the time history MAC. Experimental tests of the vibration measurement were performed, and the results of mode shape and computation time between a representative SI method and the proposed method were compared.