A strain sensor with an optical signal using a magnetic Kerr effect and an inverse magnetostrictive effect on a Fe/Mn-Ir exchange-coupled single-domain magnetic thin film was fabricated and evaluated. The optical sensing technique has the advantage of not inducing noise from external electromagnetic interference. In addition, since the proposed method using the magnetic Kerr effect on single-domain exchange-coupled magnetic thin film utilizes only utilizes magnetization rotation, Barkhausen noise due to domain wall pinning can be excluded. The strain sensor was consisted of a linearly polarized He-Ne laser and various optical devices such as a prism beam splitter and a differential amplifier as well as exchange-coupled magnetic thin film. The compressive stress was measured with the strain sensor in SiO2 (40 nm)/Fe (50 nm)/Mn-Ir (10 nm)/Fe (50 nm)/Ru (1 nm) exchange-coupled magnetic thin film. The sensor output voltage was proportional to the applied compressive stress of σ<100 MPa in the strain sensor. The sensing range and sensitivity to the stress in the strain sensor was also easily changed by varying the Fe ferromagnetic layer thickness.