Recently, many technical developments aiming generation of above 500 GPa, such as the double-stage diamond anvil cell (Dubrovinsky et al. 2012, 2015; Dubrovinskaia et al. 2016; Lobanov et al. 2015; Moore et al. 2018; Sakai et al. 2015, 2018) and the toroidal diamond anvil cell (Dewaele et al. 2018; Jenei et al. 2018), were reported. Although the equations of state (EoSs) of gold (Au), platinum (Pt), tungsten (W), and rhenium (Re) were used in these studies to determine the experimental pressure, the inter-comparisons of these pressure scales at multi-megabar pressure are poorly understood. Moreover, especially for Re, there is a discrepancy (large pressure mismatch) between the previously reported EoSs by Dubrovinsky et al. (2012) and by Anzellini et al. (2014). In order to explore the consistency of the EoSs of Re and those of Pt and/or periclase (MgO), we performed simultaneous volume measurements for MgO and Pt, and for Pt and Re up to about 300 GPa using a conventional diamond anvil cell with a double beveled 40 μm culet. Powder X-ray diffraction patterns were obtained at SPring-8 BL10XU. In the case for MgO and Pt, samples were annealed by using a laser heating system to minimize the deviatoric stress. When we choose Keane EoS of MgO (Sakai et al., 2016) as a primary pressure scale, newly developed Re-EoS quite agrees with the Re-EoS reported by Anzellini et al. (2014) up to 300 GPa. Equation of state is, of course, strongly influenced by the pressure value. Although it is difficult to determine “absolute” pressure scale, the community should share the volume-volume relationship between pressure scale materials in wide pressure range as a “pressure scale free” information. It enables us to intercompare the primary pressure scales, and can be used to construct the internally consistent scales.