The reflectivity of a telescope primary mirror is one of the fundamental parameters that determines the telescope performance. Due to a lack of suitable instruments, however, measuring the absolute value of the reflectivity, in particular wide spectral measurements in-situ, has been almost impossible. To solve this problem, we developed a portable spectrophotometer called the Subaru Portable Spectrophotometer (SPS). SPS covers a spectral range between 380 and 1000 nm with a resolution of 2 nm. Its dimension and weight enable in-situ measurement on the primary mirror. A modified V-N method is applied to SPS for obtaining the absolute reflectivity. A sequential measurement makes SPS compensate the instrumental drift. The great advantage of SPS is its capability of getting absolute spectral reflectivity in-situ, even after the primary mirror is mounted on a telescope. In the case of Subaru Telescope, SPS clarified the reflectivity of the primary mirror coated with aluminum 4 years ago. Periodic measurements have been on-going since the primary mirror recoating in 2017. It is now possible to study the telescope reflectivity degradation with SPS.