An OTP microcomputer of the present invention includes: a PROM 14 having a terminal for receiving signals from a voltage supply Vpp that allows a program to be written therein; a MOS transistor to be used as a resistance or a resistance cable containing a pull-up resistance R4, and an input terminal 11 connected to both the terminal of the PROM and a logic circuit of the microcomputer and connected with the drain of the MOS transistor or one end of the resistance R4. In this arrangement, when a microcomputer mode in which the OTP microcomputer is operated as a normal microcomputer is selected, a voltage Vcc (.vertline.Vpp.vertline.>.vertline.Vcc.vertline.) of a device power supply provided inside the circuit of the microcomputer is supplied to the source of the MOS transistor or the other end of the resistance R4. On the other hand, when an OTP mode in which the PROM is written in with a program is selected, the voltage Vpp of the power supply for allowing a program to be written in is supplied to the source of the MOS transistor or the other end of the resistance R4.