Left: IRX–β diagram of ALPINE galaxies with previously determined relations at two different redshifts. Blue and red points show individual FIR continuum detections at 4 <  z <  5 and at 5 <  z <  6, respectively. Open downward triangles show 3σ upper limits of individual IR nondetections. Stacks are shown as blue (at z ∼ 4.5) and as red (at z ∼ 5.5) rectangles. The nondetection of stack is indicated by a downward arrow. In the bottom right of the left panel, a horizontal bar shows the median uncertainty of individual β measurements (Δβ = ±0.18). Solid and dotted lines show the IRX–β relation of local starbursts from Meurer et al. (1999) and that assuming SMC dust attenuation (e.g., Prevot et al. 1984), respectively. We also plot the updated local relation from Overzier et al. (2011) (dotted line). Assuming bluer intrinsic β, Reddy et al. (2018) proposed an IRX–β relation at z ∼ 2 which has a SMC dust extinction curve and bluer β (dot-dashed line). Right: fitting results to the stacks at each redshift assuming an intrinsic β₀ of −2.62 are shown with red and blue lines with 1σ uncertainty (orange and blue bands). Small points and downward triangles show representative stacking detections and upper limits of z ∼ 3 galaxies, respectively (Bourne et al. 2017; Koprowski et al. 2018; Álvarez-Márquez et al. 2019; Fudamoto et al. 2020). Both at z ∼ 4.5 and z ∼ 5.5, our stacks show lower IRX than z ∼ 3 results at fixed β. The IRX–β relation from Meurer et al. (1999) is not consistent with our results. Within uncertainties, at z ∼ 4.5 and at z ∼ 5.5, our sample prefers the IRX–β relation from Reddy et al. (2018). Our stacks and fitting results indicate that galaxies follow an IRX–β relation most similar to an SMC type of attenuation.