Provided is a preventive, progression inhibitor or remedy for a disease one of the causes of which is the activation of the P13K/AKT signaling pathway or vice versa. A phosphorylation-inhibiting and/or dephosphorylating agent, which has an effect of inhibiting the phosphorylation at least at one of the phosphorylation sites of PTEN protein selected from the group consisting of T382, T383 and S380 and/or an effect of dephosphorylating the same, is prepared. Alternatively, a phosphorylation-inhibiting or dephosphorylating agent for PTEN is screened by a method comprising a step for confirming an ability of a test substance to inhibit the phosphorylation at least at one of the phosphorylation sites of PTEN protein selected from the group consisting of T382, T383 and S380 or a dephosphorylation ability thereof. Then, a substance having an effect opposite to the inhibition of PTEN phosphorylation or dephosphorylation thereof, e.g., an antibody against the phosphorylation-inhibiting or dephosphorylating agent for PTEN obtained above, is also obtained.