Reaction of 2-butanone (1) with tert-butyl nitrite (tert-BuONO) was performed using base catalysts (RO^-M⁺: R=CH₃,C₂H₅;M⁺=Li⁺,Na⁺,K⁺) in alcohols (CH₃OH or C₂H₅OH). In this report, the effects of M⁺ of RO^-M⁺ on the nitrosation mechanisms were investigated. The yield of E-hydroxyimino compound (5E) increases much better in the reaction using Na⁺ or K⁺ as M⁺ compared with that using Li⁺. It is also observed that the yield of 5E increases by addition of crown ether as a cation-capturing agent. The experimental results suggested that under the conditions lowering the effects of M⁺ of RO^-M⁺ on the nitrosation mechanisms, because the reactivity of naked enolate of 1 increases and the reaction in the C-N bond formation process tends to proceed via open-chain transition state without M⁺, the yield of 5E tends to increase.