Observed spiral galaxies are distributed on a unique plane in the three‐dimensional space of luminosity L, radius R, and rotation velocity V (hereafter scaling plane), and distributed in a surfboard‐shaped (particularly elongated) region on the plane. In this schematic figure, we use the I‐band absolute magnitude M_I(mag) for L, face‐on isophotal radius R_23.5 (kpc) for R, and H I line width W₂₀ (km s ⁻¹) for V. The well‐known scaling relations (L‐V, V‐R, and R‐L) can be understood as oblique projections of the surfboard shape. The scatters of these three correlations can also be unified by the scaling plane. We hypothesize (1) that the two‐dimensional distribution implies the existence of two dominant physical factors in spiral galaxy formation and (2) that one of them is more dominant than the other because of the surfboard shape.