We have developed an easy-to-use electrostatic force microscope for operation in ultrahigh vacuum (UHV), based on the scanning Maxwell stress microscope (SMM), for application to surface science and nanoelectric devices. The SMM head is built on an UHV-flange with setting all the optical parts in the air side. This design makes operation of our UHV-SMM as convenient as operating an instrument in air. The stable oscillation at very small tip-sample separation without persistent adhesion under UHV opens up the possibility to improve the lateral resolution of the SMM.