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We investigate the probability distribution of the spins of primordial blackholes (PBHs) formed in the universe dominated by a perfect fluid with thelinear equation of state $p=w\rho$, where $p$ and $\rho$ are the pressure andenergy density of the fluid, respectively. We particularly focus on theparameter region $0<w\leq 1/3$ since the larger value of the spin is expectedfor the softer equation of state than that of the radiation fluid ($w=1/3$).The angular momentum inside the collapsing region is estimated based on thelinear perturbation equation at the turn-around time which we define as thetime when the linear velocity perturbation in the conformal Newtonian gaugetakes the minimum value. The probability distribution is derived based on thepeak theory with the Gaussian curvature perturbation. We find that the rootmean square of the non-dimensional Kerr parameter $\sqrt{\langlea_{*}^2\rangle}$ is approximately proportional to$(M/M_{H})^{-1/3}(6w)^{-(1+2w)/(1+3w)}$, where $M$ and $M_{H}$ are the mass ofthe PBH and the horizon mass at the horizon entry, respectively. Therefore thetypical value of the spin parameter decreases with the value of $w$. We alsoevaluate the mass and spin distribution $P(a_{*}, M)$, taking account of thecritical phenomena. We find that, while the spin is mostly distributed in therange of $10^{-3.9}\leq a_{*}\leq 10^{-1.8}$ for the radiation-dominateduniverse, the peak of the spin distribution is shifted to the larger range$10^{-3.0}\leq a_{*}\leq 10^{-0.7}$ for $w=10^{-3}$.

Spins of primordial black holes formed with a soft equation of state